Insert MIME type of an URL into Apache’s access_log

Those looks like attempts to leverage mod_proxy to get at other parts of the 'net. An incorrectly configured mod_proxy config makes a web-server an open proxy, very valuable for people trying to look like they're not from the place that they actually are. I can't be sure, but that's sure what it looks like to me.

Apache mod_proxy documentation, on access-control:

http://httpd.apache.org/docs/2.0/mod/mod_proxy.html#access

A few things you can try:

• Try setting EnableSendfile off in the Apache config and restart (see Apache core documentation). Also try EnableMMAP off (see here for a issue very similar to yours).
• When testing with a browser make sure the cache is cleared and/or force the image to be reloaded.
• Test with a variety of file sizes and types. Is it just PNG files, all images, etc...? Narrowing in on what image size works and what doesn't might help in the end.
• Try testing another web server (lighttpd, nginx, ...) and see if they exhibit the same behavior. This should tell you if the issue is Apache specific or something on the server/network itself.
• Download files both remotely and locally and see if the response is any different.