Ideally pictures should not be hung above radiators. Extreme or rapid changes in temperature cause paper and wood to dry out and adhesives to fail.
Damp can cause pictures to ripple. If the ripples touch the glass, the picture might stick and be hard to remove. Damp also encourages fungal growth - likely to show as brown stains. Conservation framing can slow these effects, but it is always best to avoid hanging framed pictures in humid conditions. Allow six months before hanging pictures on newly plastered walls.
Remember most pictures are designed to be viewed at eye-level. When hanging a group of pictures of different sizes align the top edges. Groups of pictures need not be hung in symmetrical patterns, but they should follow some sort of overall design. Try arranging them on the floor first.
Use two hooks on the wall, each set about a quarter of the way in from either side of the picture. Check that the cord, wire or other hanger you use is designed to support the weight of your artwork. Where safety is critical, in children's bedrooms, for example, ask me about security fittings and glazing.
A gentle clean
Dust frames or treat with a soft brush, rather than risk applying water or cleaning fluids. Don't use cleaning fluids or water on the varnished surface of oil paintings; again dust carefully. If cleaning fluids have to be used on the glass, apply them to a duster first (rather than spraying the glass directly); take care not to let the fluids touch the frame.
When transporting and moving pictures always pick them up with a hand on each side. NEVER pick them up by the top edge alone.