I know I usually bang on about how much better it is to work from life.......and it usually is for all sorts of reasons........but I also recognise that there are people who simply cannot do this, for all sorts of other reasons. I remember having a bit of a row with a tutor about this some years ago; he claimed that all work done from photos was abhorrent. (He also thought art instruction books were abhorrent. He was a strange chap) When I pointed out to him that there are people who are simply physically unable to work outdoors and yet might want to paint landscapes - should they be denied this opportunity? He huffed a lot and said they should stick to still life painting. I thought he was being rather horrible. Why shouldn't people paint whatever they want to paint, and if they choose to work from photographic reference - so be it. However, it is important to recognise that sometimes, photos have limitations, and one of these limitations is that the camera cannot always expose for both the light and the dark areas of a scene at the same time, and the photograph can sometimes be rather inadequate.
I have done crits for art groups, where a big dark patch has appeared in someone's work. When asked what it was, the person would defensively claim "well, it was there, in the photo". In other words, they had no idea what was there. so they just painted the dark patch - dark.
BUT there is a useful tool for those with a computer and access to Photoshop - I use Photoshop Elements, which is the vastly cheaper version of this complex bit of software. Elements is pretty user-friendly, and can be put to excellent use for improving inadequate photos.
Apart from the "crop" tool, which speaks for itself and will obviously be useful for composing from a photo, there is a very handy little tool which can actually show you what is going on in the shadows! See the barn at the top of this post? Well look at this...same photo, adjusted in Photoshop: