specifics of effect types:
- Create image would work like a three dimensional hologram. That would make it somewhat harder to work with (especially its inability to selectively hide things without covering them with something), but it would make it work on multiple targets and avoid the need to get through any mental defenses the target might have (defenses they might have if they were, say, an
a hunter of evil wizards).
- Hide image would work like a mirage or
http://www.wired.com/dangerroom/2011/10/invisibility-cloak-mirage/ by bending light. This is similar to making the area invisible, but has the disadvantage of creating a visible blur (which gets more noticeable when covering large areas and when seen up close) and the advantage of not being seen through when detected (if it is detected they will know something is hidden, but they won't be able to see what without without additional magic).
- Create noise would be fairly self-explanatory. The spell produces noises like a speaker.
- Hide sound would work by creating an invisible barrier that slows and smooths movement of air in and/or out of it, functioning sort of like hide image except with sound.
The general idea is to use magically driven physical effects rather than directly magical ones, to make the spells easier to conceal from detection and to make them more difficult to counter once detected.
Direct mental effects are easy and quick when they work, but most of the beings you
really want to hide from have equally formidable mental protection.