I'm in the process of studying my photographs, choosing which are good reference for paintings and which aren't. There are a number of questions that need to be asked in order to make that decision.
Is there a center of interest, main subject or "star" of the composition?
What do I want to say with this image, what is the main idea or message?
Is the lighting interesting or is there a full range of values?
What is the palette?
Do I have a good understanding of the space and forms?
If the answers are mostly yes then I will take the time to do thumbnails and color studies to further work out the composition, colors and values.
I find this exercise a good gage of how far I have come in my ability to communicate visually. One very good indicator is how dependent am I on the photo to develop the image. Essentially it's necessary for me to truly understand the subject matter so that I am not a slave to the photograph. Without that understanding the photo becomes a crutch and the resulting painting is often quite mediocre.