You can save c4ds anywhere and open them with cntrl o. For the lighten stuff check this topic out. http://www.gamerenders.com/forum/index.php?showtopic=225708 I wonder who posted those pics? lol
As u use GIMP, do NOT set ANYTHING to lighten-trust me on this Turn it to "Screen", "Overlay", or just play around with the mode settings
i'd either save them as psd, png, or targa format. Either way, the lightening blending modes work out in any format IMO.