To be honest, I learned how to use Inkscape and Gimp, and made all of my own map assets (I'm also a content creator). Inkscape works well for laying down lines and text, and for getting tiles to "snap" to one another seamlessly. Gimp is great for textures and colors. Generally what I do for dungeons is create a bunch of rectangles of one color to represent chambers and then I add thinner rectangles between them to represent halls and tunnels. Each of them is made with 100 pixels per square (and I later resize everything to 70%). I export it all to a PNG file named "Floor". I go into Gimp and import "Floor". I go around each edge with a brush and make the walls slightly irregular. I select all of the color of the floors, and create a layer above that and call it "Walls". I increase the selection by 5 pixels, and paint everything a new color on the Wall layer. I then reverse selection, and increase by 10 pixels. I then reverse selection again and delete it all. I then create extra layers in between, and brush textures over the areas of floors and walls, and delete everything outside of those boundaries. That's the basics, you can then play around with Drop Shadows, various brushes, and a number of different textures to get the look you want.
