I recently updated our app from plain screenshots to including a caption and device frames and did some looking into this... there are a ton of options, but I eventually went with appure and have been happy so far.
Name/URL : https://appure.io
Pitch: Create and localize screenshots for the App Store or Google Play. Android, iPhone and Ipad.
More details: 25+ layout combinations ( text above/below device, frameless mode etc.), 28 languages with non-latin fonts, Custom fonts ( upload your ttf's), Bold, italic, underlined captions ( pretty much like bbcode); Basic plan is free, so it give it a shot.
Looking For: General feedback and early adopters
Discount: Just ping me at for a discount