I don't use a website to upload pics. Make sure you are hitting either NEW REPLY or QUOTE or else you won't be able to add an attachment. Does it say at the top of the "attachment page" INVALID something something? BF- this has been doing this for me as well. Try copy and pasting your pic to the post as well. Sometimes that works for me too.
...and on that day when my strength is failing, the end draws near and my times has come, still my soul will sing Your praise unending, ten thousand years and then forevermore!