i want to decorate the jwm window and tray with pixmaps, instead of just simple colors
Too bad. You do have a couple choices. You can use one color if you want things to be flat; you can use two color values separated by a colon if you want a gradient. http://joewing.net/programs/jwm/config.shtml#colors
If you want more than that, try another small window manager that allows theming with image files -- oroborus and enlightenment both allow that. I don't consider enlightenment, with its quirky and peculiar libraries, to be "small."
EDIT: FWIW, jwm is a lot faster sans icons. I removed them from my DSL menus (I run three root menus) and got marginally faster response. I also removed them from the default menu in Vector Linux and am getting noticeably better performance -- the toll of 60-something mixed scalable and 48x48 icons is pretty steep, and removing them from the menu (and getting rid of their ugly bloated wallpaper) resulted in nearly halving jwm's RAM use.ok, i'll check out icewm, its sometimes used in puppylinux so it wont be that fat.
Of course, the more crap (fancy graphics) you throw in it the bigger it is. I've had KDE 3.5 run very efficiently on older hardware using smaller icons and turning off transparency, no wallpaper, etc.doesn't fluxbox do pixmaps?