There is nothing that will keep them for long. Nothing that doesn't include yearly re-doing. Even decks that are wood have to be sealed and resealed. I decided long ago no garden art that is glass or wood. Broken glass in my garden is dangerous and wood just means it will need replacing with in two years.

A good quality outdoor grade latex paint will keep your window frames in good shape for several years as long as you maintain it. Picture frames are mostly made from a lower grade of open pore wood that will absorb any moisture that gets past the paint. using several coats on frames should help.

What about using spray on polyurathane? It says for indoors on the label but I have used it successfully on things outdoors.

As long as you go into the project knowing they won't last forever, then I would either paint them with an outdoor paint or seal them with a polyurathane spray and put them out. They will give you several years of joy and by that time, you may be ready for a change!