It could be a number of things. Tech problems has already been mentioned.
The picture that's not showing on MySpace, is it located on MySpace or your personal web space or is it somewhere else. Sometimes when you try to post/link to a picture that is not on your webspace, you can get a red cross or sometimes another image. This is because the host has something called "Hotlink protection". Hotlink protection makes sure that your pictures can only be displayed on a site that is specified as allowed. So unless the website has said it's OK for these pictures to be shown on Myspace, they won't appear.
A good example of this are Tripod sites. if I try to add a picture from a Tripod site by using the following code:
Code:
[img]http://members.tripod.com/~tyratyra/img2.gif[/img ]
It's not going to work. I'll get the "this image is hosted by Tripod" image. Which looks like this...
But if you copy and paste the link into your address bar, it's fine -
http://members.tripod.com/~tyratyra/img2.gif
Hope my rambling made sense.