Trail textures  right-click on the picture you want and choose "Save Picture/Image As...", save to your "Armagetron/moviepack/" folder if you have moviepack installed or to "Armagetron/textures/" if you dont. The files need to be renamed to dir_wall.png to use them. I'd suggest backing up the default file you already have to dir_wall.org.png or something similar and copying the one you want to use and renaming it dir_wall.png (this way you can have all the files available for future use). The pictures below look best in Mozilla (true .png support) :)

Wings  simply love these :) all except the last have 75% opaque pattern and low stripe (the last is completely transparent except for fully opaque pattern).

Hearts  by request kinda lol :) all except the last have 75% opaque pattern and low stripe (the last is completely transparent except for fully opaque pattern). This texture has the added benefit of being directional (shows which way opponent was moving).

Transparent with low stripe at bottom  easier to grind with because of the low stripe.

Plain transparent 

Other transparent textures 

Iceman has made his own transparent version available at his site (unlike mine it makes all trails have black glass on purpose).

locutus has also made some transparent textures and these are directional :) they can be found at this location.

Texture problems  there is an issue with colors that are similar to the arena background being way to difficult to see, by adding nontransparent patterns/designs on top this is at least patially solved. None of us has yet figured out a way to remove the color strip at the top.