You should probably start a new question, since your problem has nothing to do with the first question. You should provide your image loading code, so that we can see what internal format and pixel transfer formats you're using. Something is definitely wrong with the image data being uploaded

Download gDEBugger and see if the textures are actually being uploaded

If it works with DDS-loaded textures but not JPEG loaded textures then your problem is almost certainly in your image loading code, not the shaders or texture binding.

@MSalters Yes, floating point operations are often slower than integer operations, but that's irrelevant - 0.4 and 0.56 could just as easily be converted to use integer operations. Furthermore, on modern x86 hardware, most floating point operations (other than FDIV, FSQRT, and other transcendental functions) cost essentially the same as their integer versions: 1 or 2 cycles per instruction.

@congusbongus It probably has a name, but I don't know what is is. If not, perhaps one day it will be called the Crist Distance (hah... probably not)