The images are from table reads for season 5, and the one that's got people wondering is for "Chapter Eighty: Purgatory". The comic art shows Betty Cooper, Veronica Lodge, and Jughead Jones standing over a grave, meanwhile an image of Archie in a helmet hovers over them.
It could be a reference to the fact Archie was planning on joining the Naval Academy after graduation and given we know thanks to Lili Reinhart there's going to be a significant time skip, perhaps it's a signal he's died in combat?
According to E!, the actual comic panel is from Archie 1941. In that comic, Archie was presumed dead in battle in World War II but was actually alive.
It's hard to tell if the show might play with this plot point and have Archie join the military and 'die' at some point during the time skip... after all, we know Riverdale doesn't mind a fake death (like Jughead's 'death' earlier in it's run). That said, KJ Apa has talked about leaving Riverdale before and he does have some new projects lined up so it's not totally out of the question that Archie will actually die in Riverdale's fifth season.
We still have a while to wait until these episodes arrive in 2021, but until then we'll be poring over these clues and fan theories.