Are you trying to start a gallery, or are you trying to create an avatar?
The term "Profile picture" usually refers to a gallery. Information about starting a gallery can be found at this link:
http://forum.lowcarber.org/showthread.php?t=236445
If you're trying to create an avatar, i.e. the little picture that shows beside your nickname on each of your posts, there are size limitations. The image must be 100 x 100 pixels, or else 19.5 KB,
whichever is smaller. In my experience, even when I resize a photo to 100 x 100 pixels, I still have to set the file size to be 19.5KB or smaller in order for the image to load here.
I don't know about this forum, but for other forums I belong to, only certain file extensions are acceptable for images. Usually jpg, jpeg, png, and gif will work, but I have the best success if I use jpg or jpeg. If you're using Photoshop to resize your photos, be sure to use "Save for Web".
There are a lot of free photo resizing sites online. I already linked
in this thread to one called "Shrink Pictures", which I often use. (I used it to make my current avatar from a pic of one of my holiday cacti.) If you don't like that one, just google the words
resize pictures, and you will find many other sites.
If this information doesn't help, perhaps if you were to post the steps you follow when you try to post the image, someone will be better able to help.