I've seen a couple of posts lately where people are having problems uploading attachments.

This latest version of vBulletin keeps all your attachments that you have loaded in the past, so I figure it has something to do with that.

What I have found, when you start a thread and want to attach a picture to it, you select the "Manage Attachments" button and a new window appears. This is where your past attachments will start showing up as thumbnails.

The trick is, close that window, and click on "Manage Attachments" again, and everything should be fine.

I know its a pain, but its the workaround I have found to add attachments to a post.