Couple questions about some of TD's tiles, From the desert tileset, in particular Options

[DeathRay2K]

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...