Hello.jpg was probably the first ever really famous internet meme - bigger than "All Your Base" and I'm sure it was even bigger than "Rickrolling".
Hello.jpg was hosted on a site with the address goatse.cx - it's pronounced "Goat Sex".
This meme was made so famous because of its shock value - it was widely used in Internet Relay Chat where links were easily cloaked so a user had to arrive at the link's address to know where it actually led. Links in IRC were made to look like they were going somewhere useful but actually took the person who went to it to the Goat Sex web site.
Even though the site's address was pronounced Goat Sex - the shock value of the site didn't have anything to do with having sex with goats. The shock value came from the picture named hello.jpg being prominently displayed in the center of the page and taking up a large amount of real estate on the user's monitor.
The picture hello.jpg showed a naked man stretching his anus with both hands, to approximately the width of his fist. The inside of his rectum is clearly visible. If that's not descriptive enough for you - ask someone with a very large mouth to open their mouth as wide as they can and to put the back of their tongue up so you can see it - it looked something like this but an even larger opening and of a human being's anus.
It seems today that the Goatse.cx web site is still alive and kicking - but the owner of the domain is peddling the Goat Sex Coin crypto-currency.
Although there is no hello.jpg to greet you - you can visit the web site here: goatse.cx
