If anyone is interested in studying these stamps, there is a wonderful book by David Parker - European Stamp Issues of the Second World War. It's a good read, because it puts the stamps in their historical context. Poland was split up. Parts of it ie Posen - were annexed by Germany. Parts were taken by Russia. The remainder was run by the General Government. The new masters very quickly replaced traditional Polish imagery with German imagery. Often this was quite subtle - so that images of Cracow castle were used, not because of its Polish past, but because it was the base for the new German-backed government.
Here are stamps produced in 1943. They are not worth much, but they are startling well made. At a time when resources of paper and printing materials were in short supply, the Germans felt that expenditure on stamp production was a amjor priority.
Why Polish Stamps are Worth Collecting