We are aware of the problem with new forum attachments not working.
In a nutshell, the database that stores the attachments have gotten a little too big and unwieldy. Too... many... pictures...
Rather than throw more space at it or take out valuable development time to redo the entire thing, we are going to try to implement a more reasonable policy to keep the attachments from consuming too much space.
For now, please stay tuned. We will be removing some of the older attachments once we arrive at a definition of "reasonable."
Please keep in mind this only affects images you post directly- those get stored in our forum database. If you put the picture elsewhere on the Web and link to it in your post as a URL, this is, naturally, unaffected.