If the background is really black, or really white and the image in the center is exactly the same, there is no difference in the size of the .jpg file. I tried both maximim and minimum quality settings and both result in the same size file whether the background is black or white. If you use a drape or piece of paper as your background, there will be detail in it which will influence the size of the file as the JPEG process tries to reproduce it.
Conclusion: don't worry about influencing the size of the file you create. Use black or white as you prefer.
http://www.thechipboard.com/cgi-bin/bbs521/webbbs_config.pl/read/695147
http://www.thechipboard.com/cgi-bin/bbs521/webbbs_config.pl/read/695169