MicrostockGroup

one has to wait just a few days the first time, and longer and longer each time if one is failed again

I would save at highest possible quality for test (100% in Corel PhotoPaint, or Level 12 in Adobe Photoshop);
Then, for normal submissions, I'd save at 97% or Level 11 to save some bandwidth without reducing quality in a sensible way.

Image size is >= 3 Megapixels (but most other sites are >= 4 Megapixels, so it's better not reducing too much)

Choose 3 radically different images for the test, to show them that you are ready to shoot photos in many categories (e.g. people, food, architecture, and so on)

The quiz is quite easy instead, they ask the usual questions about image quality, noise, releases...