Please do not
1) Upload covers separately when you can replace them with "Upload a new version of this file" (The images are cached and it will take time for thumb to generate)
2) Replace already higher quality covers with the lower res Steam ones. Only exception is if the previous cover is littered with logos or has abnormal aspect ratio.
3) Use random names for the images. We always use "Game name cover.jpg" or .png
I have cherrypicked what covers were better but majority of them were deleted, because they were either worse quality or totally different design.