WED works better with a 256x256 pixels texture (.bmp .Tga formats)
but you can use one from 4x4 < image < 1024x1024
In Photoshop:
To resize the image go to: Image-Image Size
Then save as Targa or Bmp
I am trying to import my textures to WED, but there nothing!
There are 4 panels: Object-Texture-Default.wad-Bookmarks
Do you know why there are the functions of the 4 panels?
and how do I import my textures and apply to 3D models?
Christian already explained it above, man; I had the same question. After reading what he said, it took a few minutes for me to experiment with it, but I figured out how to do it. (I think this is what you're asking)
Go to the
textures folder on the right menu. Click on
default.wed, and then right click on the single texture that's in there (it should be grey bricks). A list should appear. Click on
texture manager. Then, click on
New Wad. Create a wad (a wad is merely a folder for textures, I made a wad entitled "woodtextures", for all of the wood textures I upload). Then, your new wad folder will appear with the other ones (object, texture, etc) on the right menu.
Open your new .wad, and right click in the black area. Click on the "add texture" option. That's as far as I got tonight, thanls to Christian ^_^ Hope this helps you.
Christian, I can't thank you enough, man. I've taken your advice, I scaled down the snap to grid function (I didn't know that I could, but am greatful that I can), and it works perfectly now. The thickness of my walls of about 1/4 the width of a box, so I adjusted the snap grid function to that measure, and it's working perfectly.
I haven't done what you suggested in GIMP yet, but it's getting late, so I'll probably end up doing it in the morning. I'll let you know how everything goes, and again, many thanks! 
-Kravenwolf
