The guest (and staff) graphics are in the same format as all of the graphics within ZT. There are image files representing the direction they face, such as E, N, NE, S, and SE. There are also color files, such as man.pal. Since these are in the same format as everything else, the ZOOT program can be used to extract the images. The images have been extracted and used before. For example, the "Guest as an Animal" download extracted and used them.
However, if you are trying to make new images that you want to be used as new guest images, that is more difficult. The reason is that Zoo Tycoon will change the colors on the fly. For example, one guest will have one hair color, another guest will have a different hair color. So man.pal has one of the hair colors. But Zoo Tycoon will sometimes change that and use one of the colors in hair.pal instead. Therefore, if you were making new "man" graphics, you would have to use the same colors that are in man.pal and no other colors. More importantly, you would have to use a graphics program that put those colors in the color palette in the same order that they are in man.pal. At this point in time, we do not know how to accomplish that. If we could do that, we would be able to create all downloads more efficiently than we can now since we could use a single ".pal" file for each user created item instead of multiple ".pal" files.
But sometimes you can get close. When I created the Gorilla As Guest and Animal As Guest hacks, I reduced the number of colors that would be substituted to 1. That still causes some pixels here and there to change, but not as many. I tried to reduce the number of substituted colors to 0, but that caused the game to crash eventually.