Image Upload Problem

If you try to upload a image or insert an avatar, sometimes it keeps saying, file type is not allowed. I dunno, some JPG images work, some GIF images work. Some don't. How do i fix this? My members are having lots of complaints about this. Here, read this. It's a sample:
http://echat.forum0.net/request-f17/avatar-req-t10.htm

Bump. Still need help.

Hello,

I dont know whats going on there, but I do recommend that you upload your image to a image hoster (such as servimg or photobucket) and copy and paste the link into the appropriate place.

Also, were not aloud to double post within 24 hours

What images are you trying to upload - format, size, dimensions?

Just some random images. Whether it's JPG or GIF. Some work, some don't. Do you know any causes?

What are the dementions of the images that you are trying to upload?

I dunno. I just upload some random stuff when I feel like it. They come in varieties of sizes. Just please give me some possible causes. Thanks.