Just some supplamental information.
When the image stops rendering, I do not see an error in the report or the Red X that has been reported by some users, I just see a blank picture box with the border showing but no image. I have tried closing and re-opening the browser with no difference. I also tried in Edge and Chrome and both display the same issue at the same time and both start working again after the SSRS Service restart.
I have checked the SSRS Log file and no errors are reported, just INFO messages.
The server is quite busy during business hours however, images stopped working several times throughout the working day, therefore, it is unlikely the server is releasing the SSRS site from memory due to lack of activity.
Currently the memory usage is sitting at approx 5GB with the images working in the SSRS report however, just an FYI, this is NOT the SQL serve that hosts any databases. It is only used currently for it's SSRS capabilities. The actual databases are hosted on a separate SQL 2005 Server. With that said, there doesn't appear to be any problems retrieving the database records because data relating to the records are shown, it's just the image that doesn't render.
I am happy to post the log file if it will help. Any suggestions would be greatly appreciated.