http://i.imgur.com/lclOtpu.jpg
The image on the left is small ui size and right is normal ui size.

Clearly the screenshots displayed when starting scenarios or challenges were optimized for the normal UI size however the small UI size is default and on this setting the images appear aliased ranging from "Showcase: Infantry" where the screenshot simply looks aliased and as if it was taken in a small resolution without anti-aliasing and in "COF: Green" it makes the text harder to read.

This can be remedied by changing the image size to be the same size on all UI settings or by using a better method for downscaling the picture sizes on smaller UI settings.