I'm going to weigh in on this one because I feel it really needs to be addressed with the whole 'pictures' subject and for good reason: I'm a photographer.
1) Megapixels help no matter how you look at it though realistically 8mp is more than enough to see the results for our needs, which everything on the market today would be overkill.
2) Zoom isn't going to do as much as the lens' minimum focusing distance. You want to PHYSICALLY get close to your subject.
3) Zooming into the subject increases the f/stop which requires either longer shutter speeds or higher ISO speeds, the latter giving poor results as there is an increase in noise.
4) A macro lens would be ideal however these cost an arm and leg but would also not be able to give you 'the big picture' shot, you'd only get fine details.
5) For objects like the back of the head to determine hair regrowth I'm fairly sure we'd be fine with a regular 18-55mm kit lens most SLR cameras ship with today, you just have to set the f/stop to around f/5.6 or higher as it will produce a sharper image (lower always means softer images) and the bokeh is less obvious, as in, how much is in focus, in which case, at f/5.6 on a relatively flat object up close is fine.
Tips to get your photos turning out the way you want them to (for the purpose of this thread)
1) Use soft lighting as opposed to bare light bulbs or god forbid, on camera flash. Best way is to find a room that has a lot of windows and natural light. Sometimes long fluorescent lights high above with the semi-transparent covers work quite well as the covers act somewhat like a diffuser. You want *soft* shadows not the hard shadows that on camera flash produces
2) Get as close (physically) to your subject as possible, zooming in won't hurt but won't matter much.
3) Theres a general rule that your shutter speed should be the same as your zoom length. Thus, if your shooting at 50mm, you'll need a shutter speed of 1/50th of a second or fast. If at 100mm zoom range, then 1/100th of a second shutter speed is necessary. This is because nobody has perfectly still hands and you'll often find slightly blurry photos because of your hand shaking. This is normal. So, just make sure your shooting at a fast enough shutter speed to get your subject (back of head and hair) as still as possible.
4) If your using an entry level digital SLR, you'll need to factor in the crop factor. Essentially what this means is that entry level SLRs have smaller sensors than regular 35mm film, therefor zoom ranges are 1.6X or 1.5X as much, ie; 100mm becomes 160mm. That means, you'll need to shoot at 1/160 sec. if at 100mm.
I hope this helps.