Its not "illegal" actually to post ANYTHING in Usenet, but depending on your usenet provider, they can shitcan your account.
Usenet servers are all held in Cornell, it was the first method of internet communication, the only people that actually own it is Cornell University, and trust me, if they flip the switch all of usenet goes down.
Most providers though as long as you "stay on topic" wont care what you post unless you do a MASS flood.
Tell yea, if your promoting your OWN site, slip a FEW images in a group that deals with that subject , put your url on the bottom of the image, if they want to see more, theyll go to your site.
Dont be like Naughty Jenn though and spam all 600k newsgroups with 12 naked pictures of you all in seperate postings and cross postings to 5 different groups.
Imagine going to your favorite Well baby newsgroup to chat with all the other mothers and you see Naughty jenns ugly mug staring back at you ass in the air with her crusty corn hole spread.
That will get you shitcanned VERY quickly.
Best spamming Usenet service I know of that is the most lax is
http://www.easynews.com