Did you click on the "Attachment Manager" link right under your post? All work is stored in a tmp directory until you choose to post it.
The error page says that you already have the 15 images attached. Just close the message and work on the post text or submit your post.
If you want remove a previously uploaded image, click on the "Attachment Manager" link again, and remove an item from that list. You will be presented with the "15 items limit error" again is you try to attach another one in addition to the maximum items youve already attached.
