Is it legal to copy other people's pictures?

[Orest3228]

Hello! = ) Help solve the problem.
In the World Wide Web has a lot of different portals, desktop wallpaper . All the wallpaper is in the public domain. You can upload any in any format. Suppose I open a small shop and do this shop portal . then take one of these oboin , edit, cut out a piece , such as the horizon, and pour to your web site .
Will it be a violation of copyright ? The idea image is in the public domain and I take a piece of the whole picture. I just extract profits from the pictures (that I do not sell the picture .)I make a profit from the store. A picture is just a web site . Can make a claim for it ? If so, who? Will it be a violation of the law ?
Thanks in advance if anyone can help to understand this