Use either Paintshop Pro or Adobe Photoshop. Jasc Paintshop pro has a free trial for download at their site. Then you either need to find a nice image on google that you can crop and re-arrange or be a very good artist. After that just crop the image to the correct size and then covert it into 256 color mode. Then save it as a .pcx or if you are making a leaderhead or unit save it as a .flc in Animation Shop. Then you can edit the pediaicons to display your new graphics or just replace the new with the old.
That was probably not exactly what you were looking for but your question was rather open ended. Making graphics can take time and you first need to become comfortable in using a program. If you don't want to put the time into it then post what you need made and maybe someone will do it for you. Otherwise start reading tutorials and messing around (backup files first!).