with attribution

http://breezi.com/blog/using-placeit-images-for-commercial-p...

You’re completely ok to use PlaceIt-generated-images for commercial use. You just need to attribute PlaceIt if the work is publicly displayed. We’re ok with any attribution so long as you’re clearly not trying to be an asshole and just burying it. Doesn’t have to be large or very prominent either. Anything along the lines of mentioning PlaceIt is fine. We’d obviously really appreciate a link back.

Hmm, I certainly won't 'watermark' my app store screenshots and distract potential customers. I'd rather pay a fee to breezi for using the templates.

You can mention that you used them in the app description.