There appears to be a small problem with images on posts. After a while, Multiply deletes the original image, and stores a reduced sized copy on the message. I will verify that this is indeed the case with customer support and not a bug, but if you ever post a message with an image, make sure that in addition to inserting the file as an image, you insert it as an attachment, so that we can always retrieve the original. (I had to edit all the Penmanship images because of this issue.)
From what I can see, this only applies to images. Attachments have not been affected.