Alright, so the site that Simkl gets TV show data from, TheTVDB, has recently undergone some changes. Since those changes, images for TV episodes are no longer appearing for some shows- old images that have already been there stay, but any new ones added on TheTVDB will not appear on Simkl.
One example of a series with this problem would be MasterChef. On Simkl, the latest image shown for season 10 is "Someone's Toast"(https://simkl.com/tv/20473/masterchef-us/episodes/#s10). However, on TheTVDB, the latest image shown is "One Pan Wonder" (https://www.thetvdb.com/series/masterchef-us/seasons/10).
An admin from TheTVDB responded to me saying, "If any images are broken since the rollback, it's likely because those sites are still using the 302 redirect that we had in place for https://www.thetvdb.com/banners to http://artworks.thetvdb.com/banners. This is something we cannot fix from our side. Most sites and systems are handling images properly."
Hope this can get fixed soon!