I'm in a slump. I have been going through literally thousands of photos for ideas to work from, but nothing is inspiring me. Its made me realize something that the photos do and do not do for me. When I take photos for art work reference, I am often sketching the scene, and I am smelling the hot coffee or exhaust, hearing the birds chirp or the ocean waves rolling and its the responses of all my senses that contribute to creation and form an idea in my head.
When capture something digitally, I can work from those images with very fresh memories of the location. But once those memories fade, the photos are useless to me. So I've realized I need to get out and about more - I've been too much in my house. I've gone sketching more this past week, and that has generated an idea for a new project - more on that later.
The other thing I wanted to mention about photography for painting reference is that its different from photography as art or as documenting family vacations, etc. I went to France and Italy over a decade ago and I don't have a single photo I can use as reference. I took them all as a tourist not as an artist interpreting for painting. I look for unusual cropping, light and shadow, layers, energy and movement for painting reference. I see the painting in my head and record it digitally. I literally shoot from the hip sometimes because even if the shot isn't good, it will bring to mind that image that made me click the shutter and then I can paint what is in my head.
I suggest to beginners that they don't try to paint from magazine photos or pretty calendar pictures. Those images are already art and the photographer looks for something that won't translate into paintings.
Another issue is the alterations the camera lens makes to the images our eyes see. Charles Sovek is a favorite of mine and he has some interesting lessons on his site - one about painting from photos here. Amazon has several pages of books about painting from photos and I have this one very good one, How Photography Can Make You A Better Painter by Michael Weymouth that discusses the technical aspects of HOW TO photograph specifically for painting reference. Scott Burdick also did a video lesson on using Photoshop to enhance pictures for painting. All very good resource materials.