3.14
3.10
Those pics are already in .tga (targa) format. Photoshop, Gimp and similar programs can save on that format, or you can use small image viewer such as IrfanView to transform between types your preferred editor can read. Don't use jpg or gif, because they approximate some of the data and that breaks shadow and other special colors.
Pure black (0, 0, 0 in RGB) is totally transparent, magenta (angry pink) is unit's shadow.
Magenta has two numeric values all the way to the top, it's (255, 0, 255) IIRC.
The sprites go into \dominions 3\mods if you link to them as #spr1 "monster.tga" or into \dominions 3\mods\urquan if you link to them as #spr1 ".\urquan\spathi.tga"