The options "Never replace cover images with smaller ones" and "Never replace selected cover image types" do not work as expected. The are implemented as filters (bigger_previous_image_filter and image_types_filter) and as such are run when loading cover art from sources.

For this to work the release must have files attached already at time of loading. Smaller images won't be loaded in this case at all. But if the release has no files attached the filters don't apply and cover art gets loaded regardless. If the user then assigns files to the release images of smaller size or wrong type will be replaced.

The filters should be run when copying loaded images over to the file metadata instead. Likely this means that the filters also must not be applied during loading time (even if it could save some downloads, but later assigning different files will not give the expected results).