You do not need to have .BMPs as long as you are using Soldat PolyWorks (since MapMaker, MapMaker Plus, and probably all the other, older editors do not support .PNG). If you are not using Anna's absolutely brilliant
Soldat PolyWorks, you'll need to convert the .PNGs to .BMPs. Paint will do the job (open the graphic in MS Paint, and "Save As" the filetype .BMP).
Put the file(s) in ..\Soldat\Scenery-gfx, and if you are using MM/MM+, just quit the program and reopen it. For PolyWorks, just click the small arrow in the top-right of its Scenery window and refresh the list.