There are a number of reasons why pictures will not display.
One is the hosting service you use for the pictures. (ie. Photobucket) Sometimes the hosting will time out, or the server will be down for maintenance and that will cause the picture to not display.
Another is vBulletin, the forum software, has some quirks. One of which is when you edit your post it tends to remove the image. This is an error in the software that hasn't been addressed yet by the company.
is the proper code for images. You can also use the Enter Image button in the post box (yellow logo w/ mountains and sun on it).