custom module shows double 'blank' images?
Design and Skin Customization
Yeah nothing in the docs, but break it once and you get a nice message:
Here is a small image of me

https://www.jamroom.net/user/image/user_image/4/small/crop=auto
and with a non-existant image size
https://www.jamroom.net/user/image/user_image/4/super-massive/crop=auto
The second one produces the useful error:
Quote: CRI: invalid image size - must be one of: 24,xxsmall,40,xsmall,56,72,small,96,icon96,128,icon,196,medium,256,large,320,larger,384,xlarge,512,xxlarge,800,xxxlarge,1280
You'll see they line up with what SteveX has provided above.
If you do manage to find error messages that are not useful, let us know.