You will really enjoy the highlands and the cultures you find there. The people are very open with photographers and usually don't expect compensation. I travel with a backup polaroid so that I can leave them with photos of themselves as a gift. That is ALWAYS a big hit and well worth the extra weight.
From the coastal regions to the highlands you will find the children will fight for the chance to be photographed.

Always give them a chance to see themselves if you can. They love seeing their pictures.

While they don't expect compensation, a pocket full of "lollies" (candy) goes a long way, too. DO get a guide who knows the area and DO follow what the guide says. They will know when things get a bit dicey and will steer you away from problems. You never know when a tribal tiff is underway that could mean a certain road should be avoided or a village should be bypassed. We narrowly missed a machete slaying in the Inga provence during a tribal "failure to communicate". We saw several newly burned huts in areas with a feud going. PNG is a highly litigous society where any wrongdoing is a situation demanding compensation from the offending clan. Any failure to make proper compensation is a reason for immediate and sometimes violent confrontation. So, listen to your guide.

Ask your guide what village he is from. Often they are very happy to take you to their village and show YOU off. Sometimes you will have to hike off the road to get there, but you will see the true PNG, off the tourist track, if you do. Once, we had to drive down a section of road "belonging" to a tribe his village was having a feud with. It caused some tense moments. Luckily, no one from the other village saw us using "their" section of road.
You will have several villages perform their tribal dances and songs for you.

Be aware that the songs often have lyrics made up on the spot. If your guide starts snickering, be sure that you are the subject of the song. One of our songs was a lament about how they would be working in their kau kau fields if they didn't have to entertain us....! That's OK. We sent them a VHS tape of their performance. They will be able to find a machine, possibly at one of the local lodges who will allow the village to watch the tape. It will cause a real celebration when they all get to see themselves perform.
As far as clothing taboos. Be aware that they don't mind seeing a woman topless, but they don't appreciate a woman showing anything between the waist and knees. Thighs should not be visible. Be careful about wearing shorts or bathing suits away from the lodges. keep a lap-lap (sarong) close by to throw over yourself. I only wear knee length shorts or long pants with no problems. Informal wear is OK everywhere, and t-shirts are fine. If you carry a bunch of t-shirts you can afford to lose, leave them with your guide before you leave. It is HIGHLY appreciated and they will be put to good use.
All in all, you will love it! I will be back for a month this Feb/Mar, but will not be getting to the highlands this time. Use care and common sense and you will be fine!