As various Plex apps request images from your Plex Media Server, they will be generated as needed and then cached for future use.
The specific location where transcoded content is stored varies based on your operating system:
On Windows, the content is located inside of the user directory.
%LOCALAPPDATA%\Plex Media Server\Cache\PhotoTranscoder\
Using the Finder's "Go" menu, select Go To Folder... then enter the following in the dialog box that pops up:
In general, the location for the various Linux versions of Plex Media Server will be found under:
$PLEX_HOME/Library/Application Support/Plex Media Server/Cache/PhotoTranscoder/
/share/MD0_DATA/.qpkg/PlexMediaServer/Library/Plex Media Server/Cache/PhotoTranscoder
/apps/plexmediaserver/MediaLibrary/Plex Media Server/Cache/PhotoTranscoder
/Volume1/Plex/Library/Application Support/Plex Media Server/Cache/PhotoTranscoder