just my opinion - something in a dk or sport weight with good stitch definition works well for illusion. nothing fuzzy or hairy. the idea is to get the purl bumps to really stand out "above" the knit "V"s so the picture is visible.
also, I wouldn't recommend worsted weight if your chart has a lot of rows. for each line, or slice, of the picture, you will create 4 rows (2 in which the purl bumps stand out and 2 in which the knit Vs lie flat). so if you're using something as thick as worsted, your picture will be huge.
I used Rowan Cashsoft 4-ply for my Decepticon. check my blog thread for a pic.
I think Lion Microspun will work well.
good luck, HTH