If you're talking about Lemmy, it depends on what you're using to browse. I use boost and for images like that, at the top right of when I have the image open there is a "HD" button which loads the image in full quality. Take a look around the UI when looking at an image, you may be able to find a button like that. Another option is to go into settings and look for an option to always load images in the best quality, that would also probably help.
Good luck ๐