For most of these you can adapt an existing .dat file, so you'll need to change the lines that begin 'emptyimage' to refer to your images.
So if you were adjusting the HSTXC.dat file (the CrossCountry HST), you would change every line like this:
EmptyImage{s}=HSTXC.0.0 (I changed the square brackets as they introduce strikethrough on the forum)
to
EmptyImage{s}=YOURFILENAME.0.0
You can do this with a find/replace. This will also change the names of the vehicles. Your vehicles need to have a unique name, i.e. not be the same as something else already in the pakset, or they won't show up. You also need to pay attention to the Constraint lines in the .dat, which tell you what can come before/after that vehicle. If you do the find/replace method this should all be taken care of.
Next you'll need to find a version of makeobj (and maybe pakhelper) to help turn the dat and png into a pak folder. Does anybody know where the most recent versions of those are kept?