Here are some rocks from Pixabay that you can download and use without needing to give credit:
https://pixabay.com/vectors/mountain-hill-view-peak-rock-575619/
https://pixabay.com/vectors/stone-rock-nature-pebble-zen-576268/
https://pixabay.com/vectors/rock-boulder-stone-nature-granite-576682/
Download the smaller version; copy, paste and resize to fit your map.
Find or make some forest images, then arrange them as needed on one of the base maps, but put that on a layer below the grid so that the grid shows over top of the forest.
Make 2 or 3 buildings and save them as independent small PNG files on clear backgrounds. By making different sized boxes, you can rearrange them to make more interesting shaped buildings.