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.
...I am not skilled to understand, what God has willed, what God has planned, I only know at His right hand, stands One who is my Savior....