Hi Rita- Yes, you can use frozen bread for croutons, and it's best to cut the slices into cubes before baking in the oven. You do not need to wait for the bread to thaw before making croutons with it. Good luck!
Thanks Ken! Yes, I know that I should cut into cubes before the baking, no worries :-). I was just wondering if it would be easier to froze it already in cubes or just slices, but after your answer, now I know that either ways works fine.