If you’re not a sacristan/assistant sacristan, server, priest, or deacon, you shouldn’t be putting anything on an altar at church. Altars are sacred. Don’t get handsy.
Now, putting stuff on the altar steps – that’s closer to okay.
Putting stuff down on the floor in front of the altar but before you get to the steps (or somewhat away from the altar if the altar is on floor level)? Totally okay for laypeople without special altar business.
But even if you are a priest, and you’ve been assigned to run Divine Mercy devotions in church, it usually would not be proper to put a Divine Mercy picture on the altar. An altar cross is the decoration that belongs up there.
But putting a little table in front of the altar with the Divine Mercy picture? Fine.
Or making a beautiful “frontal” picture or cloth covering up the altar legs? Also totally fine.