As various Plex apps request images from your Plex Media Server, they will be generated as needed and then cached for future use.
File Locations
The specific location where transcoded content is stored varies based on your operating system:
Windows
On Windows, the content is located inside of the user directory.
%LOCALAPPDATA%\Plex Media Server\Cache\PhotoTranscoder
OS X
Using the Finder’s “Go” menu, select Go To Folder… then enter the following in the dialog box that pops up:
~/Library/Caches/PlexMediaServer/PhotoTranscoder/
Linux
General
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/
QNAP
/share/MD0_DATA/.qpkg/PlexMediaServer/Library/Plex Media Server/Cache/PhotoTranscoder
ReadyNAS ROS4
/c/.plex/Library/Cache/PhotoTranscoder
ReadyNAS ROS6
/apps/plexmediaserver/MediaLibrary/Plex Media Server/Cache/PhotoTranscoder
Synology
/Volume1/Plex/Library/Application Support/Plex Media Server/Cache/PhotoTranscoder