Normally you tap the text of a comment to make the vote buttons appear, but if the comment is only a picture, clicking it will just open the picture. I've only been able to vote by tapping the username and then finding the comment from their profile. Let me know if I'm missing something.

Edit: I've found that tapping text to the right of the username (the score or the time since posted) brings up the vote buttons, so this seems to resolve the issue.