Tile/Map-Based Game Techniques: Handling Terrain Transitions might help.
If you have tiles (e.g. grass textures, dirt textures etc) you might want to do a search for 'texture splatting' which shows how to use the graphics API to blend them together.

Those links above are really good stuff - I've used one or two of those tutorials before in some of my projects. You can also look into using alpha-blended transparency and spike-diffused edges in a PNG image - I never perfected the technique but my tests showed a lot of promise.

For instance, allow 10-20% (depending on your tile size) of the tile's edge on all four sides to blend away with alpha transparency and then rough up the edges with spike diffusion (so its not just a pure fadeout). When you assemble the tiles on the map, have an overlap of 10-20% so the faintest edges of the lefthand tile is covering the part of the righthand tile where that tile begins to fade. The advantage of this (besides automatic seamlessness) is the transitions from grass to dirt tiles are natural looking. (The spike diffused part of the edge will look like clods of grass or dirt, depending on the tile) Ditto for sand, snow, etc. Obviously you'll need some angled edge tiles, because otherwise it will be a blocky looking mess, but the transition itself will look good.

Like I said, the tests I ran looked swell, but I never got it perfected enough to use in a game...so take it for what it's worth. Maybe you or someone else here can do something with this novel idea and make it work.
-Jake