Issue with Images' Caching

[Mko]

Hello,
One of my fellow Administrators has been reporting an odd issue he's been experiencing with my Forum dealing with the caching of images. I've looked around a bunch and can't seem to figure out the cause and how to fix it.

First thing's first: my forum is in a subdirectory on my server (for example purposes, let's say it's in /forum/).
Now, my main page works perfectly fine (aka http://mydomain.com/index.php), and any images updated there do not require my Admin's cache to be emptied (according to him, at least). Now, in my /forum/ subdirectory (http://mydomain.com/forum/), things change.

Below are some examples he has experienced dealing with images' cache:
-When I added custom statusicons to forum sections (using custom prefixes), he was not able to see the new statusicons unless he cleared his cache.
-When I updated the whos_online.gif and stats.gif images in the /misc/ directory, he needed to clear his cache for them to update for him.
-When I tweaked the code in the memberinfo page (two templates specifically: memberinfo_block_ministats and memberinfo_visitormessage) to display avatars not in thumb format (thus without the white background put on transparent avatars, and also without dateline) using only the URL image.php?u=$prepared[userid] and image.php?u=$message[postuserid], respectively, when he changed his avatar, he needed to empty his cache. An odd occurrence was if he replaced his big (100x100) avatar with a smaller (20x20) one, his old avatar image would remain, but it would be shrunken to a 20x20 size.

From what I can conclude, it seems in general any images I change in my /forum/ subdirectory require him to empty his cache for them to update.

I, myself, have never experienced this issue as I normally do not save cache or history on my browser.

To sum up what I need help with: What is the cause of this, and is there any workaround?


Thanks a bunch,
Mark/Mko