Thank you for your post.
I am unable to replicate he problem on both Mac OS X 10.5.8 and Mac OS X 10.6.4. In each case, imagemac does work for me.
Have you tried this with a fresh database file (not another table in the same database file)? What plug-ins do you have loaded? What version of Mac OS X are you using? Any other information you can provide may be helpful in narrowing down this issue.
If this still fails with a new database file, I would like to see your file. Please check your Inbox at the top of this page for instructions where to send your database file along with one of your JPG files.