Whenever you upload a graphic to Roll20, it automatically creates 4 other versions at different resolutions (thumb, med, orig and max). This is so that a graphic that is "very close" to the needed zoom size is available, and keeps the system from trying to serve a 1000px wide graphic when the display size is a 70px token, for example. This is very similar to apps like Google maps, which store multiple map tile sizes for this very reason. This is why graphics might sometimes appear with different quality at different sizes. Roll20 automatically converts all of your uploaded graphics to pngs, to reduce compression artifacts that can occur from jpgs. (If you uploaded a jpg, only the original jpg upload size is counted in your storage quota, thought, so that's cool). All that is background to explain minor changes in appearance. What you are posting though is not minor, and seems to be outside the range of normal behavior. Without a lot of testing, it might be difficult to figure out where the degradation is coming in. It might be from a badly-cached image, and a reupload or browser cache dump might help. It might be something that happened during the upload process, if the original was a jpg, for instance—this one is unlikely, but possible, and you might be able to mitigate by deleting the original from your art library and re-uploading. Another possibility to try is to find the most common display size. If this is a map or token graphic that goes on the VTT, this might be difficult, due to the nature of zooming, but it if is displaying in a journal or handout, you might be able to decipher the final display size and upload a graphic that is specifically already that size. These are some suggestions. The amount of degradation you are posting looks way outside what is normal. My best guess is that your browser is being served the wrong size image, probably due to bad cache in the browser or interference from Amazon Web Services, which hosts all of Roll20's asset serving. Try in a different browser that you don't normally use for Roll20, or a re-upload before anything else.
