I've got an "text" image in the center of a PNG page.
I want to crop the image, removing the white space above, below, and on the sides.

This means I need to set a threshhold to get rid of any "noise" around it.
Tools->Color Tools->Threshold

Photoshop has a "crop to pixel" that would cut the surrounding white space.
Gimp has Image->Autocrop Image.

One problem I have is that my image is 96 DPI and I need to make it 300 DPI.

So I should be able to Select->All, then create a new Image, setting the X and Y to 300 DPI, then paste in the image. Right?

At that point, how do I script it all?