Light source -
http://i169.photobucket.com/albums/u...ghtsource1.jpg
http://i169.photobucket.com/albums/u...ghtsource2.jpg
http://i169.photobucket.com/albums/u...ghtsource3.jpg
(Of course, you adjustment gradient layers over the light source to get the soft colours to suit your sig)
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High res image - This is just how big the image is.
Say you have a mini image (10px X 10px) and you stretch it to 1000px X 1000px your going to get a pixelated mess.
So, keep to big high quality renders. So you can work with them better.