They could have just been graves he did out in the wastelands after the bombs fell as a memorial to them or they could have even been people he found and buried out of respect. Hard to tell but it seems like they wanted the to represent people important to him.
That is what I was thinking too, maybe victims from the war.
The original beginning for RR has Max approach a farmhouse, only to see Wez and the boys attacking the farm family and hanging them from a tree. They see Max and he takes off down the highway. Filmed but not used.
That would be cool to see.