Unfortunately not. I have this problem in another wiki as well even with the ones that are frequently replaced there (or at least I have one other wiki where I can see a drop in quality compared to other browsers). I already tried to look for a solution to this online but all the suggestions (hard-refreshing, deleting your cookies, etc.) never worked for me. Now that I think about it I gotta say that I only seem to have this problem with images uploaded to wikia, though. So you're probably right about it being related to how the files are displayed on different browsers
The weird thing I just noticed, though, is that other sprites in this wiki seem to be perfectly fine in Chrome, quality-wise (or, at least, the Siyat dragon sprites are). The Magma sprite - according to IE - shouldn't have a drop in quality either so my best guess now is that maybe the image hasn't updated for me/in general yet - I think we all know that wikia sometimes has that problem. If that's not the case ... well, then I'm stumped
You can't replace files if they are of different types - the Magma Sprite in the article used a gif, the one I saved from the site itself was a png.
Anyways, according to my Internet Explorer the sprite's fine now. The quality in Chrome is still terrible (I can hard-refresh all I want, for some reason it just doesn't seem to like wikia files <_<) but that's not the fault of the sprite itself
I see the same problem with the Magma Dragon's Sprite in this wiki (despite refreshing and trying different browsers). I'm also kind of surprised that it's in gif format because when I saved the Sprite from my scroll it was a png. Here, I uploaded the png version. When I view it in Chrome the quality still sucks (but that's not surprising, since that's a general problem with Chrome on this site) but if I look at it with IE it's fine