Images2tga may not save, but as I recall (though in truth it's been several years since I used irfanview, and matters may have changed) irvanview will overwrite the ~.tga file. Itr may mean different things; for example, in these forums a "PM" may be a "private message"; however it's a "progressive mesh" file, an older and now obsoleted file format used by Trainz, having been supplanted by the indexed mesh (~.im) format. A "~.pm" file is most likely not editable; to make changes one would have to have access to the original files from which it was mad (probably 3DS MAX, GMAX, or 3D crafter; probably little content in ~.pm format was made in Blender). Note that some assets are textured with other filetypes that ~.tga files. For example, the asset "Ferry boat Sophie 38 v3 drivable", <kuid:524343:6146> on the DLS is textured with ~.jpg files, so one would need to edit and overwrite the relevant ~.jpg file to be able to reskin it. If you open a file which is originally a ~.jpg, and save it as a ~.tga file will save fine, but the mesh will not see the new texture.
It might be useful if you'd post the asset name and KUID of the asset you're trying to skin, so we can be all on the same page.
ns