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.
If you’d like to contribute a photo to be used as a PlaceIt environment, please submit it here.