Modular design is key here; you want to create tiles similar to how WotC did their map tiles. Small, individual tile sets that you piece together like a jigsaw by either dragging into the browser or by uploading them into your art library and dragging them from them to create the maps as you go along through the dungeon (or piecing them together before the session and hiding via FoW). This allows for much more control over the design of the dungeon, and even allows you to create maps randomly and completely on the fly using some form of random dungeon design rolls. Throw in some preset encounters for your system (Respect the Ecology! :D) and you've got a dungeon you can blast together in no time and as the game is progressing. In terms of creating the actual dungeon tiles graphics themselves, you would have to use an external program. I use Photoshop to create various style tiles, dark caves, light caves, desert, snow, temperate, ect and then drop them in. They follow the typical '2x2', 2x4', '8x4', ect theme. You can do this with simple textures with very little changes by expanding them into a ratio that equals the size you want. Because I'm fussy, I create documents that are 70 x number of squares of the room, so for a 2x4 corridor, I'd create a 140x280px document and just paste the texture in. Figure out the size of your room in squares, or however you measure your system, ensure that checks up with the measurements with the page your working with in Roll20 (so a 15' by 20' room with each square being 5', you'd have 3x4 squares, multiplied by 70, or however many pixels you wish to use). Once you've done that, the rest is as simple as scaling it up in size to match the required grid size you use. If you stick to 70px per square (which is the default size for the Roll20 grid) then you can easily drag and drop. If not, you can set the dimension yourself easily.
