Likely, I have a fair amount of contact with random internet people (running game servers), seeing quite a few spam emails from y7mail.com; notably from people I have previously had direct contact with (so I am in the accounts address book).
You can go look it up, but IIRC, the slingshot method (using a rare alignment of planets) produced a higher "speed" (so to speak) than would be reasonable to produce even with today's technology. (i.e taking the same budget today and doing everything we can to get a probe out at the same "speed" would be impossible).
It seems they have judged the potential for "free" marketing to be worth the risk of a backlash from the 4chan community.
Given they are in the business of allowing users to post content, the risk/reward looks to be heavily weighted towards having their service (in the best case!) being spammed with shocking content 24/7. They haven't thought this through.
SVGs tend to be smaller because it is a vector format. It has nothing to do with XML; it seems highly likely a binary encoding for SVG would be massively more efficient than xml.