There is a tilesets and graphics subforum, but that is more for releases.
Do you mean where to put graphics in the Dwarf Fortress folder, or where to upload pictures to share?
If it is just to share a picture, you just need to upload it online somewhere, then link the image's url. It can be linked as an image using the image tags, which look like this:
[img]www.imagesite.com[/img]
Graphics in the DF folder go in the raw/graphics folder. It is a bit more complicated than normal modding, and looking at preexisting examples from other modders may help you understand how it works.
Both the image and the text file that defines graphics go in this folder. You can create subfolders, if you want. Like other raws, you will need to give the file a proper header, though you will also have to define the image to use.
graphics_x
[OBJECT:GRAPHICS]
[TILE_PAGE:x] < This is the ID of the tile page
[FILE:folder/image.png] < This is the image used, just enter the image's name and file type. If it is in a folder, enter the folder's name first, then a slash.
[TILE_DIM:18:18] < This is the pixel dimension of the tiles that make up the page
[PAGE_DIM:6:10] < This is how many tiles make up the page, both horizontally and vertically