Both maps actually look pretty good.
Photoshop has a feature that creates a shadow around an object - Gimp probably has something similar.
Try RobA's advice and create a shadow or black outline around the wall. I think that you will be happy with the result.

Personally, I am old school enough that I like solid black walls, but there are a lot of maps that want to depict wall materials and most of the better ones are made or broken by the wall shadows rather than the pattern (IMHO).

I think that the grass texture is scaled too large and the marble floor needs something to suggest tiles rather than a single slab. Perhaps a fine grid or pattern with alternating tiles filled with different marble patterns (or the same pattern rotated).

All these are just ideas ... ultimately it is YOUR vision that needs to shine through.