Apache + External IP from LAN + random image = nothing

First let me say a bit about my configuration:

• Apache working on PC-1 at port 80
• PC-1 has a static IP
• DMZ in router set to IP of PC-1

When I try to access any image on my webserver using the exteal IP of PC-1, with any of my computers in the same LAN of PC-1, they won't appear.

• When I try to use the local IP of PC-1 it works correctly
• When I try to use the exteal IP, using a computer outside of the LAN of PC-1, it works correctly
• When I try to use the exteal IP, using any computer inside of the LAN of PC-1, it doesn't work

The URL of the image is correct, it doesn't work even if I try to use the direct link (example: http://EXTERNAL-IP/teste.jpg).

I also tried direct links for other content type like .rar or .txt, these both work correctly in the same situation I mention above. Pages also appear correctly (without the images of course).

Also, there are no errors in Apache error.log and I can find the GET request in Apache access.log

In short:

Only image files hosted on my server caot be displayed if I use the exteal IP using a computer inside my LAN