The texture(s) for your longsword that you want to have shiny need to have what's called an "alpha" layer (channel). This layer is a greyscale image that, in the case of longswords and other weapons, controls the shininess/reflectivity of your item model. Photoshop transparency isn't normally recognised by NWN.
In the alpha layer, white = matte/dull, while black = totally reflective/chrome. Shades of grey are partially shiny/reflective (darker = shinier).
You can use Photoshop to save the final 4-channel image (red, green, blue, alpha) as an uncompressed .tga (targa) image, which can be used with NWN (and/or converted to a NWN-specific .dds format later).