Seems like the media is the one twisting stuff. I've been searching for the actual quote from the White House official that said they cannot confirm the pictures. I've been looking for nearly an hour, no media website has provided a source at all. However, I found it. The official said that he was referring to Israeli media, but it seems that was because he did not realize the pictures were not public.
However:
The Israeli PM just posted some of those pictures on X(Twitter) due to claims they were lying about it.
Source: https://www.wionews.com/world/trigger-warning-israel-pms-x-account-shares-photos-of-babies-allegedly-killed-by-hamas-646017
I saw a few different sources showing it, as I don't have an account with x, but it seems the one I linked is the only one that shows multiple images rather than just one. It seems that the actual truth is that there were some beheadings, but the majority were "just" murdered.
I'm not trying to claim that Israel is in the right here, I'm just saying it seems Biden was not lying.