DAO modding tip: terrain textures
Individual terrain textures can't be changed directly: the image tiles get stuck together into big composite images for each area layout, and it's these that must be edited unless you're recompiling the area layout with custom terrain textures.
The composite textures are located in packages\core\env\[area layout name]\[area layout name].gpu.rim. The filenames are [area layout name]__pal ("pal" refers to the terrain image tiles being called the "palette" in the level editor) for the diffuse and specular textures (the specular is stored in the alpha), and [area layout name]__nrm for the normals. Note the two consecutive underscores in each filename. There is also a [area layout name]__hgt, presumably intended to be a composite of the tiles' heightmaps, but I haven't yet seen one that isn't just solid 50% grey (the source tile heightmaps themselves are fine).
To find out which area layout you need (since it's often different to the area name itself), open the Object Inspector for the area in the toolset -- you don't need to actually open the area for this, just single-click on the area with the inspector window open/docked -- and look under General -> Area Layout.
The Mage Origin fade (bhm600ar_fade_harrowing) textures are in packages\core\env\oth000d\oth000d.gpu.rim, inside oth000d__pal.dds and oth000d__nrm.dds.
The Dalish Origin camp (bed200ar_dalish_camp) terrain textures are in packages\core\env\brc997d\brc997d.gpu.rim, inside brc997d__pal.dds and brc997d__nrm.dds.
The composite images are compressed as DXT5 with mipmaps; the normals are swizzled (red -> alpha) and left-handed for DirectX (green channel "lit" from below).
The original tiles are uncompressed 8-bit (u8888 for diffuse, normals and specular, u888 (no alpha) for heightmap) with mipmaps; the normals are unswizzled and left-handed.
The original tiles can mostly be found in packages\core\data_tools\toolset.erf, prefixed with ter_ for the named tiles, and trn_ for the defaults, but the fade textures are in the standard textures erf (packages\core\data\textures.erf, not packages\core\textures\high\texturepack.erf), prefixed with fei_terfade_.