The basic rule for photographs and if they can be used for a painting is if you can't see it in the photo, you won't see it in the painting.
In this example the detail is fine. Any picture with as much detail as this is perfect for a portrait. Note the eye detail. A fairly close shot from an 7 - 10 megapixel camera will give a sufficient level of detail. This wedding shot was taken on an 8Mp DSLR camera and is fine for a portrait.
This next picture is an example of what can be done. The photo is 30 years old and was scanned in. As you can see from the inset that there is very little detail, but as the painting was to be of the entire photo and not a close up this didn't matter as I didn't need any extra detail.
One final example of a superbly detailed picture. This was taken with a 16Mp DSLR camera without flash in ambient light. Because the face fills the frame, it is vastly detailed and could yield a very intricate painting.
This next photograph is only just about passable. If it had any less detail around the eye it would be unusable. Although I could do a portrait from this shot, it would be suitable only for pastels or other broad/soft medium where detail is less of a concern.
This next example was also taken on a 16Mp camera. Considering how far the subject was away (and what you see here is only about 50% of the whole photo)detail is still sufficient enough to use for a fair painting. However if a close-up painting of just the face was wanted it is only just passable.
One last example (well, two really) of some full-body and three-quarter shots just to show the difference pixel-count makes. On the left we have a full-length photo which I was asked to do a painting from. It was taken on a 21Mp camera but I was supplied with a 5Mp photo equivalent of a 6"x4" print. The detail was sufficient (only just!)but only really because I knew the couple. On the right is a photo that was taken in low light with my 16Mp camera. So the conclusion is this: For a decent photo to do a painting from, try and keep the subject as large as possible in the frame, and if you can't take the photo on as high a resolution as possible.
I hope this has helped you to some degree. Any comments or questions, please feel free to contact me. Thank you.