A couple of questions... .
 

Dear Members,

Does anyone here know whether or not I can use GIMP to edit and/or create my own .lbm files for my mod?

There once was a tutorial written for an older version of paintshop pro (trial, I believe). I someone knows where that is and where I can get a copy of the programme in question (for free) then that would also be a viable option.

Finally, which of the standard empty .shp files should I use for my mod? For the most part I've been using 99s for my turreted vehicles. From what I gather 100a are better for aircraft and other turret-less vehicles, right? If I put my icons in at icon123 onwards should I use my 100s or would another size be better?

Re: A couple of questions... .
 

I follow Cross's great little guide HERE to create .LBM files.

As to which .shp files to use, I believe you need to ensure you use ones set up to the "Correct" vehicle type (by looking at WINSPMBT_MasterIconList.txt in the Design subfolder of the game data).
I use 110 for turreted, 111 for helos, 183 for "invisible" / AA Turrets and 185 for aircraft / unturreted vehicles.

Re: A couple of questions... .
 

Thanks for the help. Have started making .lbms and loading the icons into their respective shape files. Making good progress.