It was something new to me too, but I found something :D
Pixel submission is a type of url submission method by purchasing pixels from other websites. "Million Dollar Homepage" is one. Google it and you can find more info about it.
I have no idea what it has to do with SEO..
I guess it has to do with SEO because no one at all is really going to ever click on a pixel you buy off of some million dollar homepage clone site, so the only reason someone would do it is because they would do something such as spend $100 buying 100 different pixels across 100 different domains for 100 SEO backlinks that no one would ever click on.
To be honest, when I see a such pixels, I think it looks really cool but I personally wont click to it. I do wonder what is the click through rate. It must be lower than average PPC on Google Adsense, especially if its from a non-popular site.