You have posted .jpg images, which is identified by looking at the file name. These are fine as well, but JPG formats are best used when the image in question is very detailed. The volume covers are not very detailed, so PNG would retain the image quality better than JPGs without too much difference in the storage size of the image.
I will also note that you don't have to keep an eye out for PNG images of the volume covers. I have uploaded PNG versions onto this wiki today. I didn't earlier because I have real-life work to do, and I would have gotten back to this eventually. In other words, I already had a stash ready to be uploaded. I hope there are no hard feelings with this. I appreciate your help regardless.