Sewingsuz:
You may want to check to see what file format it is in, ie jpeg, pdf, txt, doc, etc.
I tried for a long time to post a photo of fabrics I want to put up for sale on the board and after trying for 1-1/2 hours to load the file as a PDF (which the manage your attachments indicates as being an acceptable format - I kept getting the same message you did).
Had to change the format to a jpeg and it went through the first time I tried (hopefully, it did - I will have to await the mandatory pre-approval for sale items before I can actually see my listing & its photo).
Don't know if this helps you - hopefully it will.