Thank you for your post.
Your setup does not sound out of the ordinary. FileMaker Server 8 only serves the files, while FileMaker Pro 9.0v3 accesses the file.
Are any other applications running? Having Mail and Safari open are expected, but I'd like to know all applications that are currently running.
Is it just this one database file? Create another database file and insert some of the same graphics from the original file. Open this on the server and check the access again. Is there a difference?
If you know, where did these JPEGs originate?
Any other information you can provide about your machine may help provide some clues and narrow the possible causes.