Try Paint Shop Pro 7 (not later versions) for .lbm editing and saving. IrfanViewer can open them and save copies, but not ovewrite them.
Lbm images are stored in WinSPMBT\Game Data\Images and icons are stored in WinSPMBT\Game Data\Graphics in .shp files, which you edit SHPed.exe stored in the same folder, with an eye on the list WinSPMBT_MasterIconList.txt which you find in WinSPMBT\Game Data\Design.
Hope that was clear.
As for how to use all that to actually make lbms and icons, you should look into the "Tables of Organization & Equipment" sub-forum above, where info about that kind of subjects is discussed. You should find enough tips in old threads to get started.
Have fun,
Plasma