Can I get copyright Issues, when I use PNGs from the Internet in a Webcomic, but I use them highly transformative?

Efficiency is everything as a Comic Artist. And it can really make some things quicker if I implement photos in some places. For example, if I used a photo of a City panorama in the background, rendering over it and seamlessly making it part of the background. Or if I used some texture from a photo. Basically doing, what people do in Photoshop to an extend. Could I get legal problems with that, if my comic suddenly became successful? Or if I started making money with it? Or is it like on YouTube, where you can use footage as long as it's transformative?