The simple answer is that they didn't burn all the bodies. They didn't even burn most of them. Most were placed in earthen pits and covered. The pictures of the mass graves were the ones they were still actively using when the camps were overrun by either Soviet our US forces. These are the pictures you see publicised. There are also subsequent excavations of mass graves that uncovered many more bodies, none were burned. I imagine if they started burning them, they probably stopped towards the end of the war because of the aforementioned rationing.
The simple answer is that they didn't burn all the bodies. They didn't even burn most of them. Most were placed in earthen pits and covered. The pictures of the mass graves were the ones they were still actively using when the camps were overrun by either Soviet our US forces. These are the pictures you see publicised. There are also subsequent excavations of mass graves that uncovered many more bodies, none were burned. I imagine if they started burning them, they probably stopped towards the end of the war because of the aforementioned rationing.