Whoa, I think calling people after only 10 mins after I've sent the mail is pretty obtrusive. Surely I would give them enough time to read the mail, think about it and give me a reply whenever they're able to (within a reasonable amount of time, say 2 hours).
I would make a special case for if they read it immediately; they will still perceive you as calling immediately after sending it since you aren't revealing that you used the image to find out that they read it.
