By default, Plex won’t display photos over 100 Mib (Mebibytes - I had to look that up when I learned about this limit). This is a server-side limit, rather than the app.
There is an override, to be used with caution, since the limit is there for performance reasons. See this page, and look for the TranscoderPhotoFileSizeLimitMiB advanced setting:
https://support.plex.tv/articles/201105343-advanced-hidden-server-settings/
