http://www.artscope.net/VAREVIEWS/MORTON0800-1.html
If you click on the link above, you will see some of the work of James Morton. He uses Photoshop alterations early in his work. There was a thread about it a few months ago on the Studio Forum. At the time I wasn't paying attention, because I didn't have Photoshop. Now I can't find it.
I think that just seeing these photos, adjusted in this manner, gives you the confidence to proceed, because it proves that they "can" become good paintings. Since I paint from photos, I plan to use the altered images at the start of my paintings. Instead of working from the realistic photo, I will try working from the "underpainting" filter, or the "watercolor" filter,which ever is better, for the blocking in part of the painting. And then I will switch to the realistic photo for the rest of the painting.
Like Debra Jones said, so many times I see a painting in a scene, and then take a photo, but the photo doesn't look anything like the painting that I originally saw with my imaginative mind. These filters help recall the original thought, that you had, when you took the picture.