As far as I can tell, if they took a public domain image, modified it in some way, and sold that on their site, it would be copyright them. They could sue you f

As far as I can tell, if they took a public domain image, modified it in some way, and sold that on their site, it would be copyright them. They could sue you for using it or whatever they wanted. What would count as modified, I don't know. E.g. Maybe they put real man hours in on Photoshop to remove some artifact and to correct the color. I don't think anyone would argue they shouldn't deserve to be paid. But if all they did was crop it, resize it or even just compressing it as a JPEG they might be fine in legal terms.