Might be the file type or how you're attaching it.
Is the attachment something you have on your computer, or something on the web? My attachment here came off of my computer. Using IMG tags is notoriously iffy with vBulletin.
This just started last week. Nothing i've done the last few times copying an image from the web has changed. Except it appearing. I replied to the remote starter thread. Pasted an image, which appeared in my reply. Then hit post reply, Nada, zilch, zip.