Oups, it seem than I didn't give you the right link, it's a coop project with Rennir...
I don't know of which white rectange you are talking but the purple outside have a purpose: I always use a carvas bigger than what it will be at the end, sometime it help for working with brushs and selection around the intender border, it's a trick than I got from one tutorial from escimo .
The link only had one, and it seemed to be one of the freebie ones rather than of your current series, but it is very helpful and instructive! I've never looked at a psd of someone else's sigs before, but it was very helpful to see what you did.
Why did you use the white rectangle and purple outside instead of just making the canvas size what you wanted, though?
There's a couple of persons than could give you better advice but, the first thing is the choice of the pictures you will work with; if they have colors than match, it help, if the artwork of both pictures are comparable it do help as well. If you think than it might help you, there'S a link to 4 of psd files of the fist 4 signatures of my current serie : http://www.mediafire.com/?z4zuj3hbxdxcma9 .Don't hesitate to ask if you have others question.
sempai, do you have any tips on how you blend your renders in the main sets? I'm doing a sig in a somewhat similar style (nature background, words, and render) and would like some advice if you have any