Doors with floating panels are not really meant to be painted, though people do it all the time anyway. By floating, I mean they move because they are not connected to the rest of the wood in the door. Since they are painted already, I would caulk all the panels in place with a high quality acrylic caulk like DAP Dynaflex 230. The panels may still move, but if the caulk is there at least that will give it some flex so that the cracking may not come back as soon or be as widespread. Expect there to be some maintenance though as the panels will always move so there will always be cracking.

Thanks Paintguy, but I have a question about caulking. I have the same problem with cracks at door panels only mine is much worse in some areas where about 2 mm shows as unpainted old stained wood. Since it is worse in winter I thought now would be a good time to sand down the cracks and old paint line that sticks up(I had painted in summer) and prime and paint the stained wood areas that are showing. Would you recommend the caulking route instead of the sand/prime/paint plan that I had? I'm not afraid of the work involved, just want it to look better.
Thanks again!

It's hard to recommend a for sure method that would work better because either way maintenance is involved. I have caulked whole houses of doors and painted them and had no cracking at all, but that doesn't mean every house is going to be fine. If the cracks in the doors don't bother you, then I would just paint the raw wood and then when painting try not to fill the crack with paint as that will just crack later when the panel moves.

Ah yess the dredded cracks...another thing to keep in mind that if you do two coats make sure the paint is VERY dry before the second coat. (as well as the caulking) If it's not, when the 1st coat continues to dry, it will crack the top coat. In that particular area the paint can be thicker and take longer to dry than you think. After knocking the cracked stuff out with a putty knife tapped by a hammer, caulk it and let that dry for a couple of days. Prime it, (I use Zinser oil) dry well, and two coats paint drying well between each. Shouldn't have to repaint the whole door either. The Dynaflex stuff is the best! Run a bead that will cover the stuff you knock out and smooth evenly with a wet finger, and wipe off any lines that may appear on each side of your finger. I have done this many times without sanding and it works well. Don't wipe out too much caulk or it won't cover the boo-boo. Worked for me.