So far, I've only looked at the desert tileset from TD, and I was wondering a couple of things. Firstly, are the purple squares important for anything? What do they do?
Also, do the tiles from sh17 and sh18 go in a particular order?

Apart from that, I've converted all the tiles from the desert tileset to pngs, combining the separate tiles in each .des file into single images, and replaced the green shadows with 50% opacity black. This is for something I'm working on, but if anyone else has any need for these, you know where to find me. Does anyone know of an easy way to automate the combining of each tile into an image? Right now I'm manually placing them...

Well, the tiles actually contain information to add empty cells to the grid. They don't contain the final terrain tile's dimensions though.
RAMIX contains a handy tool to convert them to PCX in the correct shape. It shows the tile graphically and allows you to change the X*Y dimensions with your arrow keys until it's correct, and then writes the tile in the correct dimensions to a PCX.

sh17 and sh18 are 2x2 blocks of animated water, using C&C's palette scrolling of palette colours #32 to #38 as animation.

The purple squares are probably simply some sort of test tiles. They are never used by the game.

Thanks! I've been using XCC, which has no such functionality as far as I can see, so I guess I'll try that tool.