"Photos"-a simple editing tool
If you use photos as a reference to paint, it is very helpful to do a little editing with the source before you start painting. You don't need Photoshop to do this, you can use the Photos app on a Mac to do some simple changes that will make a big difference. Start with an image that you are interested in painting and then go to the Edit button on the top right corner.
In the editing window, I almost always do three things right away, crop, enhance and filter/adjust. Cropping your image is probably the most important. Adjusting the composition before you start painting saves a lot of time and it also frees you up when you are documenting images on your camera. You worry less about getting a perfect composition because you know you can always adjust later. Although the mountains are beautiful in this image, I believe too much green and sky makes for kind of a boring painting so, I decided to focus on the red building in the bottom corner. I also decided at this point to make the painting a square format instead of rectangular because that was the shape of my canvas. It's very important to match the shape of your source image with the canvas so that transferring the image will be a lot easier.
The next thing I do is enhance the image. This lets the computer automatically make some adjustments, which generally are pretty good. I enhance after I crop so that it's adjusting color and contrast based on the new composition without extra information that isn't going to be in the painting anyways. The enhance tool is often very subtle and is great if you just want to improve the picture a bit before sharing with your family or printing for a photo album but, if you're using the photo for painting, the Filter tool or the Adjust tool give a bit more drama. Filter does it automatically and the Adjust tool allows you to change the light and color manually.
Finally, when I print the image, I try to print a color version in the largest size possible on a single sheet of paper. However, I usually have my computer open while I paint so, I can zoom in on parts of the image while I'm working.
Most computers come with a simple photo editing tool that probably has basically the same functions. It's a clean and simple way to to do studies before an actual painting. Of course, nothing beats actual paint and pencil studies, but adjusting your photos electronically is also a great way to practice composition and color study skills.