What you could try:
Make the background/rectangular first and save it as PNG.
Open a new file (500x300 or whatever size, transparent) and just drag the background onto it. You can place the render after that any size you want. (Hope I made sense! o.o)
[MENTION=5445]nawi[/MENTION] ; Ok, lemme know if you need help ^^
And yea transparent. Cuz if you use the normal white, you end up with a white background even if you save it as PNG.
The empty parts (non filled parts with your signature) will stay transparent that way.
Could you please re-upload; RELEASE[110520][ひよこソフト] サクラの空と、君のコト -Sweet Petals For My Dear- 初回限定版 [v1.01 + スペシャルディスク]
Sorry, but RG doesn't work for me.