Cysion replied in an AoE Zone interview about this request:
Why are full-map screenshots not possible in DE? Is it difficult to add? It is quite important if you want to show your custom maps.
“In the original game, the full-map screenshots were a crafty hack, implemented by Matt Pritchard to allow full map captures for their marketing team. The way the updated renderer works, this is no longer supported and quite a bit of work would be required to bring this feature back. It’s certainly possible, but we’ll have to weigh that up against other features.”