It depends. A lot of artists do manipulations and they use stock specifically made for that. For example they may use someone's background, a picture of a person, object, etc and combine them. But those images are sold/given away as "stock" and there are terms of use as well. Some may not be for commercial works or credit and a link to the creator's site is required and so on.
But as far as copying an image from the gallery here and modifying it, that it is not permissible as this is a gallery of finished work and not stock for others to use and the copyright rests with the creator of the image. I'd suggest you take a look at deviantart where many users upload free stock (with terms) for the use that you describe.