Tell us about the picture!
It was fun to do, and she was pretty excited when she finally found it. But until then, she was really disappointed with the letter - I got in contact with her a little later and she said "all you gave me was that shitty letter," at which point I hinted that there was more to it, and she got pretty excited.
I thought I left enough clues but apparently I didn't! When I pointed out explicitly there were clues in the message, she figured it out.