Well, just to throw a wrench into this scenario, as mentioned in another thread, I sent a picture from my iPhone and a picture from my iPad. Both signatures are typed with standard text and both devices are running iOS7. The picture sent from my iPhone was received as an attachment and the picture sent from my iPad was received inline. All other mail settings are identical on my iPhone and iPad, IMAP Lotus mail. And received mail through IMAP account forwarded to my Gmail account.
Is it possible that there is a difference in how the picture got on your device? Taken by your device camera vs saved from an email from someone else?
Sometimes I will email a PDF file to myself so I can save it on my iPad rather that using iFiles to copy it over. If I email a copy of that file later from my iPad, many times my recipient will only see an inline, small red box with an 'X' in it.
Lots of questions...not enough answers.