When picking photographs for artists to work from, there are a number of considerations to keep in mind. The most important thing is to ensure they are of a high resolution. In other words they need to show enough detail and not be blurry. A good test is to check whether it is possible to see individual eyelashes on the subject.
The second factor is to make sure there is a nice balance of light and shade. Ideally the light should fall from above and slightly to one side of the subject. In my picture the light falls from the top right which has given a good strong shadow to the left hand parts of the three subjects. Natural light is usually best, especially if the picture can be taken on a slightly overcast day. However, strong, direct sunlight can give a bit too much of a sharp contrast and can tend to bleach out the colours in the light areas. It's important to bear in mind that what makes a pleasant photograph does not always make the best reference for a successful portrait commission, since the requirements of an artist are different from those of a photographer.
The next thing to consider is the composition. It often a nice effect to have the photograph taken from slightly below the subject, or to have a "three-quarter" view with the shadowed part away from the viewer. Another possibility is to have the subject looking slightly over their shoulder. A direct gaze, such as I have worked with above, engages the viewer, but a slightly averted look also works and can suggest a more contemplative mood. On this occasion the subjects are smiling broadly but this isn't essential at all. A hint of a smile can contribute character though.
A group painting like this makes a nice gift and can be more economical in terms of overall time taken to produce the piece and the framing costs. Individual pictures, though, are great in term of handing them down through future generations. The usual formats for portraits, whether in pencil, charcoal or oil, would be either "heads and shoulders" or simple head studies (either way at approximately life size). An individual pictures featuring a single head would be about 20x24 inches for a head study or 25x30 inches for head and shoulders. Half or full length pictures are possible but would require a lot more time as a result of the need to paint the clothing.
If you are interested in commissioning a portrait in pencil, charcoal or oil, please take a look at the Commissions page of this site.