Ok, here are a few points:
The surface tiles contained in Surf.tree are all 512x512 pixels. I don't know where you spotted 2048 and 1024 sizes, but that should not be the case. (the only exception being the global level 1, 2, and 3 tiles, but we can ignore those for this discussion).
So if your Ascension base tiles are larger than 512x512, you need to cut them into 512x512 pieces. If they are smaller, you need to glue them together into 512x512 pieces (hopefully, they will already have power-of-2 size, otherwise it may get a bit messy).
Next, resolution. If you know the resolution of your Ascension tiles (pixel/degree), then it is easy to figure out at what resolution level they fit into the tile tree:
- level 4 (the base of the quadtree) has a resolution of 1024/360 pixel/degree.
- each subsequent level has twice the resolution of the previous level.
If you don't know the resolution of your tiles, it may be easiest to simply zoom into tileedit, until the view represents exactly the area of one of your 512x512 tile cuts (at a lower resolution, presumably). You can then read off the resolution, as well as the longitude and latitude indices, directly from the info box in the top left corner.
Inserting your tiles: This is actually quite easy. Assuming you found the appropriate tile location in tileedit corresponding to one of your 512x512 ascension tiles, you just have to convert the tile to DXT1 format if not already done, and then copy it to Textures\Earth\Surf\XX\YYYYYY\ZZZZZZ.dds where XX is the resolution level, YYYYYY is the latitude index, and ZZZZZZ is the longitude index you read off from the info box in tileedit. If you then navigate back to that tile in tileedit, it should show the new tile in all its high-res glory.
Once you have inserted the tiles at their highest available resolution, you should also add at least some of the lower resolution levels:
- glue an appropriate set of 2x2 tiles together to a single 1024x1024 tile, and interpolate it down to 512x512. All basic image processing tools (e.g. gimp) can do that.
- convert to DXT1, and insert again in the correct place in the tree directory structure.
You may have to repeat that for a couple of levels, but certainly not all the way down to level 1. Simply check that the jump from original texture to your texture is no longer very obvious, as you zoom across the transition.
You may have to adjust the water colour of your tiles to match seamlessly with the surrounding ocean (surface tiles don't support transparency, so you can't simply cut away the water areas).
Also, in addition to the surface tiles, you will have to edit the corresponding water mask tiles. I noticed that the water mask for Ascension island is really messed up.