I down loaded Photoshop to try and create images similar to this for my daughters room. I am wanting to take a Disney princess image, fill it with a scanned photo of my own watercolors, and then put another image in the skirt. I have been trying to figure out how to do this for hours and have met with no success. I am hoping someone can help.
Basically to create good composites you need to learn all about using Layers, making selections, using layer mask, and blending. Painting is an art and requires a lot of learning and practice using Photoshop brush tools and other features.
One way would be to put the Disney princess on a layer. Select the princess and add a layer mask.
Put your watercolor art on a layer above the princess, and clip it to the princess layer.
Add a third layer with the castle to the top of the stack and clip it to the princess layer.