Photo to portrait
The best photos to draw portraits from are crisp and clear professional non flash photography head and shoulder shots with the face being at least an inch in size as fine detail can be lost on smaller images, A good rule of thumb is your thumb ie' if the face on the photo is the size of your thumb then that's great.
portraits also look better when the subject is not facing directly straight foward but just off centre and with the eye's looking at the viewer or slightly past the lense or viewer.
It is also better if there is light coming from one side and above the subject because this creates good tone and depth.
All of the above are preferable but not absolutley essential as i have worked with old and fadded photo's before and the results have been very good and i know that sometimes an old worn photo is all you may have and in these cases i will have a look and tell you what i think can be done.
The size of the actual photo does'nt matter at all as long as the face is in focus and big enough to see all of the detail then it does'nt matter if the photo is a large 10"x8" or small 3"x2" but the standard 6"x4" is ideal.
Note how i've added the top of the subjects heads that had been cut off in the photo above.