Each forum member is allowed a total of 146.48 MBs to post pictures. You must have reached your limit.
You need to delete some of your previous pictures you've posted to gain back some MBs. Click your name in the upper right, then click 'My Attachments'. To the right of each picture listed is a white square box. Select ones you want to delete, then you'll see a trash can icon.
Page 1 that comes up has the most recent posts. Go back to the highest page for the earliest of posts. Those should be easier to discard.