Some random ponderings that might or might not help:
- IE and Firefox, on the same machine, can be using different proxies, so it could be a proxy thing even if it's working in FF and not in IE. Double-check which proxies are in use, are they the same?
- Is Typo3 running on the same machine, with the same PHP, Apache and MySQL environment/configs?
- Is there no problems if you store a JPEG image with WinSCP to the Typo3 www root and check it with a browser? How about the Moodle www root? How about inside a static HTML page, is there a difference to the "naked" JPEG file?
- If the JPEG works from Typo3 www root but not from the Moodle www root, you can eliminate the Typo3 and Moodle from the problem: a bare JPEG downloaded directly from the www root doesn't interact with the web app, then it's a pure config problem.
- The next step is to see what's different in those settings. If they are indeed the same, then it's just another config somewhere deeper...
- Is there a gzip module of some sort in use that IE would have trouble with?
- What about the HTTP request and response, are they the same for IE and FF? Is IE receiving the same HTTP response than FF, but still having problems with it? It could be the no-proxy settings that Moodle puts in the headers.
- Try also with static HTML pages that you load from the Moodle www root. Do they work? A simple PHP script?
My problem solving technique for the "full moon and sunspots" type of odd behaviour would be to try to build a simplest case where things don't work, and then the even simplier one where things work, and look for the difference.
I hope you eventually get things fixed. If you do, please remember to tell us what it was.