A photographer or designer can get very creative in photoshop and similar image editing program by using composited image. In other words, one image merged with another. This technique can range from removing a background from image and replacing it with a more desirable background, to fantasy images that would be unlikely to photograph in the real world. One method that I discussed in a prior blog post was to use greenscreen techniques. These techniques can be very effective, though the lighting on the set needs to be extemely even over the entire background to obtain good results.
Painting on layer masks in photoshop can also produce some great results. Depending upon the image, this can be a relatively quick process, or a very involved process with each image.
Another option is to use blend modes in photoshop. They can produce some fantastic results with very little effort. There are many articles on blend modes ranging from the highly scientific to explanations that may confuse those just exploring this facet of photoshop. The link below provides an overview of blend modes and how to start using them.