Emphasizing what Lindal said, 'Make sure that the texture COLOR is the default white. Anything else will darken (and/or tint) your photo texture.'
You need to place textures on a white prim face to see them in their full color. If a prim is dark, Full Bright will have little to no effect. You use Full Bright so the picture shows up regardles of time of day (i.e. at midnight, it will still be bright as ever).
You can use PNG or TGA, but you don't need any transparancy. You can even use JPEG (jpg), but many say you are going to see a loss in quality, however I've had no problems when I set the quality slider to maximum. Best to experiment and see what is suitable for your purpose.