No the units stay.
The in game minimap generated by WC3 is a blp. I think you're confusing lossless compression with HD.
TGA files store theh color values for every channel, every pixel in the image which takes up a LOT more space. BLP files try to cut corners and find an image that LOOKS like the original, but in a way that can be expressed in a MUCH smaller file.
Definitely go with BLPs for this, the file size of a 256x256 tga file isn't worth it.
If you take a screen shot of the minimap, what you're really doing is just taking a screen shot of a plane with a 256x256 texture stretched across it. You can accomplish the same thing in any image editing program by simply increasing the resolution; quality doesn't change.
There isn't much visible difference between a blp and a tga. It has less color depth... but that's wc3 as a whole anyways...