I sympathise. I get this sometimes as well, it's pretty painful. I'm not an expert on it but I read up on quite a few things and here's what's helped for me....
1) Saddle Angle / Pelvis angle / Back Straightness: As said above, your saddle is tilted back, this might mean your pelvis is tilted backwards and if your pelvis is tilted back, then your back is probably curving quite a bit. If your back is very curved then your neck will need to bend more than usual to get your head at a given angle, putting strain on it.
Try leveling your saddle, then sit on your bike and try to tilt your pelvis forward, this will help you have a straight back and your neck won't have to be so bent relative to the angle of your back.
2) Saddle fore / aft position: You have a lay back seatpost and your saddle is waaaay back there in the clamp. This means your position is more stretched out, again meaning you need to tilt your head up more to see the trail ahead. Try moving the saddle forward an inch or so. The general guideline is sit on bike, clipped in, with cranks parallel to ground. Hang a piece of string with a weight on it from your kneecap. It should be over the pedal axle. Of course that's only a rough guide but it will give you an idea of whereabouts your saddle should be.
Basically anything that either raises the handlebars and / or shortens the reach will help your neck. So that would be:
- move saddle foreward
- fit shorter stem
- fit higher rise stem
- fit riser bars
3) Helmet peak / Glasses: Make sure your helmet angle / peak or your glasses are not causing you to raise your head excessively to see down the trail, this will put added strain on your neck. A lighter helmet might help a bit too. I get worse neck ache when I use a helmet light on long night rides from the additional weight.
4) Head Position: I'm not sure about this but it seems that for me it's much better if I try to keep my head lower and use my eyes to look up the trail rather than raising my head to look all the time. I think the head is more comfortable in line with the back than "bent" up for long periods
5) Camelbak: As said above, a heavy camelbak will put added strain on the muscles at the base of your neck.
6) Stretching: I think part of the pain is caused by the muscles being locked in a load bearing position for long periods. Try moving your shoulders around, stretching your neck a bit, rotating your shoulders every ten minutes to give your neck muscles a break.
Hope that helps a bit...
This article about cycling posture has some really good illustrations that show curved back / bent neck vs straight back / straight neck position:
http://www.francefrominside.com/FFI Bike Posture web.pdf
Also, as recommended above, a bike fit might be a good idea.