[Photoshop] / [GIMP]
Through much practice and fiddling with settings I have found that using the Gaussian Blur on ones normal maps after they have been made makes a vastly good improvement ( even more so than a high resolution pic to use as a texture ) particulary so with tiles.
The texture winds up being much clearer and not much blur up close.
It doesn't suit painting textures but artitech type bricks tiles things like that it gives superb finishing touchup.
My guess reason for this is, although normal maps can make highly detailed maps of a texture, the game for whatever reason can't handle such high detail so one much 'blur' the normal map a bit for the high detail to look as good as it is supposed to.
Well it's only a guess and I'm rambling now so hope this helps someone.
I also have had a fiddle with layers. I have found it is surprising simple to add snow to a existing vanilla house through this. This is kinda universal I used both GIMP and Paint.NET - both free programs. Anyways I loaded up the original texture then I clicked on add new layer, then I loaded up a pic of a snow covered roof and used the magic wand to only select the snow. Next I unticked the background so only my new layer on the original texture was visable and I copied the snow selection and then pasted it to my new layer and then reticked the original texture background and well now it was looking good, so I tried adding it in niftscope and then I was like, OH NOOOS the Snow didn't match up properly! So what did I do to fix it, I reloaded it into GIMP and I selected the wonderful handy tool, make seamless, and I did this with the .dds and it's normalmap and well it looks a right treat, I was very happy.
Hope this ramble helps