Map/scenario editor question

Hello!

This might be impossible but who knows! My brother draws maps for DnD and I tried to make a scenario using one of his maps.

Problem is-im rubbish at it. Its a question of scale usually. I feel like i will have one part done perfectly, but it's waaay too big! Is there some way to - input the image of the map into the game, and have my computer build the outline of the map, and then I fill in the details?