So right now, deities are not a big part of the game and have very sparse descriptions. As far as I know, the only depictions possible right now are as males or females of the worshipper's race, or skeletons if it's a god of death or blight or whatever. But with temples coming up, it might be cool to have some more varied depictions of deities. Here are some ideas:
Animal Parts - Most recognizable would be the Egyptian gods like Horus with the head of a falcon or Anubis with the head of a jackal. I could also see centaur or mermaid like gods in charge of nature spheres. There are also some gods or at least pantheon figures who just are animals, like the various Native American animal characters or Hanuman the monkey or the Hindu cow goddess whose name escapes me.
Associated Objects - Thor holds a hammer, Hermes holds the caduceus, Death holds a scythe, etc. Hindu gods often hold multiple objects, like Shiva holds a trident and drum, Vishnu holds a lotus, chakram, mace and conch, Durga holds an arsenal of weapons in her many arms. They could also be associated with an item of clothing, for example Odin is often depicted as wearing a wide brimmed hat which hides his missing eye, and Freya wears the necklace/choker/torc Brisingamen. Gods might possess items which are purely fanciful like Zeus' thunderbolts, Christ's halo or Sun Wukong's cloud car. Gods might also possess very large items like a chariot, ship or fortress with which they are depicted. Some items should be treated as named artifacts, for example, Thor's hammer isn't just any hammer, it is Mjollnir a legend in its own right, while Zeus' thunderbolts are a dime a dozen.
Odd Body Parts - A third eye, extra limbs, a tail, wings, etc. The bodhisattva Vajrapani is often depicted as ogrelike and fearsome. A god might have a distinctive appearance like a goddess of great beauty or a god with bulging muscles. They could have odd skin colors, like Osiris is green, Shiva is blue.
Multiple Forms - Some deities are depicted in multiple forms. For example, the bodhisattva Avalokiteshvara is sometimes a normal humanoid, and at other times has eleven faces and a thousand arms, each with a eye in the palm. Vishnu appears as various avatars, such as Rama and Krishna. Krishna may be depicted either as an adult or as a child. Odin often uses disguises, or his form may change depending on his role, for example, Odin as a warrior, Odin as an old man, Odin as a shaman, Odin as the netherworld ferryman, etc. Odin also takes on multiple names to fit multiple roles, so a dwarf might depict "Odin as Draugadróttinn, Lord of Undead" or "Odin as Hangi, the Hanged One". Some gods might not have a specific form, but a specific feature, for example, a trickster god might be able to appear as a man or a woman or any animal, but always has a third eye and a fox's tail. So dwarves might generate depictions like "The Trickster, depicted as a young male dwarf with a third eye and a fox's tail" or "The Trickster, depicted as a horse mare with a third eye and fox's tail."
Mythos Reference - Gods could be depicted doing things that are part of their mythology, such as Thor dueling Hrungnir, Christ healing the sick or Amaterasu hiding in a cave.
Companions - Gods might be depicted with a person or animal that is strongly associated with them. For example, Odin riding his 8 legged steed, Sleipnir, or Vishnu riding on Shesha or Garuda. There might be a class of creatures identified with the diety, for example, Indra surrounded by Gandharvas, a nature god surrounded by animals, a good god surrounded by fairies and unicorns or a death god surrounded by the undead.
Sub-gods - Some religions have positions which are holy but not on the same level of gods. For example, Christian art might depict saints, angels, gargoyles, the apostles and even the occasional demon. Thai temples are guarded by Yaksha and dragons. The Norse had Jotuns, Landvaettir, and good ol' Trolls, Elves and Dwarves. A lot of cultures also have local deities, like a guardian of a particular city or a family might pray to its ancestors for guidance. Most are familiar with India's holy cows and there are also places where snakes and monkeys are protected. In china and japan there are 4 holy animal constellations that guard the cardinal directions and often show up in anime, the dragon, Seiryuu, the bird, Suzaku, the turtle-snake, Genbu, and the white tiger, Byakko, occasionally joined by the 5th animal, Qilin, representing zenith (more so in china and thus not so much in anime). But it's a little tricky to give credit to such creatures when there are real angels and demons roaming the world of DF.
EDIT: Adding this one to the top:
I had another thought, if there's a god worshipped by more than one race, for example, both humans and dwarves might have a god of fortresses, each race might depict that god as one of themselves, and have other regional variations. So you might see "Fortressman, depicted as a dwarf holding the golden pick Arcbad" in dwarf art and "Fortressman, depicted as a human holding the golden cauldron Arcbad" in human art. If there are demon-figures, like Satan or Loki, they might be depicted as a race that the home civ is at war with or a "common" mega-beast. So DwarfSatan might be depicted as a goblin, a cyclops or a weretapir. Civs might also treat the gods of their enemies as demon-figures. In real-life, Baal, Beelzebub and Sin were mostly positive gods in the pantheons of the ancient middle east that become demons in the Christian cosmology. (Baal did have a thing for human sacrifice...)
If dwarves associate a particular living thing with their theological iconography, they might get emotional responses to it. So if DwarfSatan is indeed depicted as a cyclops, or if DwarfSatan is associated with cyclopes, a dwarf who kills a cyclops could receive a happy thought when killing a "spawn of DwarfSatan" if that dwarf is religious, and other religious dwarves might lionize the cyclops slayer. "Urist felt happy when talking to a cyclops-slayer" or they might be more likely to depict the cyclops slaying in artwork. If an entire civilization, let's say goblins, worships a god that is considered Satanic by your own civ, you'll be much more likely to go to war with them and your dwarves will suffer less negative thoughts when slaying humans or stockpiling human corpses. Conversely, if an animal is considered sacred, let's say cows are, the dwarves might feel guilty when butchering a cow, and might consider cow items, like cow hoof earrings or a cow leather robe to be either highly valuable or completely taboo. They might get extra happy thoughts when consuming cow's milk and cow cheese and get extra value or happiness from cows as pets. And if a civ shares a god with your own pantheon, you should be less likely to go to war with them and feel extra guilty when killing fellow believers.
That could also lead to internal feuding, believers of A not getting along with believers of B or with non-religious dwarves, or it could open a way to start wars with other dwarven civs.