Well you have a couple diffrent options.
In photoshop you can change the dpi (dots per inch) it's in view > image size 74 is exceptrable for any screen resolution, you can make it more, but really no need unless you are printing your work then you would want it to be atleast 300 dpi.
and in image extractor when you are saving to .blp from targa it pops up a box for it, you can change it there.