I know that The TVDb pricing model has made them impractical as a data source for many apps. I think you were correct to switch to TMDb. I also know that the Trakt API no longer serves images in order to reduce their data traffic.
TMDb has very rigid requirements for images on their site. That is their decision and obviously they have the right to control that content. However, this means, many movies and TV shows are missing some or all images because none are readily available at the resolution TMDb requires. I have spent many, many hours adding images where they are missing. Many have been accepted, but many have also been deleted.
Since most of us are using your app on a relatively small screen, a very high resolution image is not required there. When an image is missing from TMDb (poster, backdrop, season image), it would be great if you could try to fetch one from another source - IMDb, OMDb (small fee required to be a patron), Fanart.TV, etc. Trakt does provide the IDs for movies and shows on many of these sites as part of their API.
For season images, if none is found anywhere, I would recommend using the show’s poster there.
I think a lower resolution image is far better than none. The TMDb folks disagree. The TVDb has a much more complete set of images because they aren’t as rigid as TMDb. Again, I do know that The TVDb is no longer a viable option, but their approach of accepting lower resolution images has proven successful.
Images add so much to using your app. It is always disappointing when a show is missing that.
Thank you for your consideration.