Wood to cave would be interesting, could be harder to pull off due to the vastly different fill tiles though. But basically you'd need to make a tile which seamlessly blends both terrains into one. For that you will need Paint.Net for sure. The easiest way to do it, is to place both tiles next to each other and edit the pixels so that they appear to mix. I'll make an example to help explain it better.
Okay, not the greatest pixel art, but you get the idea. Having a direct cutoff between the two tiles looks bad, but by overlapping the grass and rocks with the wood to break up the abrupt endings of both tiles, you end up with a more seamless merge between surfaces.