I have a painting on wood. Which is best to seal it, polyurethane?

Wish I knew the alternative that you were going to suggest. A god brand name polyurethane is the only coating I would recommend. If you were thinking of lacquer it uses solvents which may soften and change the paint that you spray it on.

It depends if the painting is in acrylic or in a water-based paint.

Water-based painting:

http://homeguides.sfgate.com/seal-handpainted-furniture-24184.html

Acrylic painting:

https://www.quora.com/How-can-I-protect-an-acrylic-painting-on-wood

you want to use a non-yellowing polyurethane. There are indoor and outdoor versions.

it depends on where you are using the piece.

Keep in mind, polyurethane will 'yellow' over time. You may want to try a spray that is non-yellowing.

A polyurethane sealer would be recommended, think carefully if you want a matte satin or gloss final finish. I used the water based minwax on my custom painted cornhole boards.