What's an ImageAssetID, and how would I use it?

An image asset ID is the asset ID of the actual image of a decal. If you minus 1 from the numbers at the end of the URL of a decal, you should get a decal image. If you don't, keep doing that until you do.

If you copy those numbers at the end of the decal image link and put them on the end of a link like this:

rbxassetid://NUMBERS (Where NUMBERS is the decal image ID/asset ID)

Or like this:

http://www.roblox.com/asset/?id=NUMBERS (Same as above)

You'll get yourself the asset link to the decal, which can be used with items such as decals, textures, imagelabels, etc.

This method is the manual way to put a decal onto a part in Studio. The other way to put a decal onto a part is by using the Toolbox in Studio.