it's a yahoo domain. You can look at the registration information here: http://www.whois.com/whois/yimg.com
and you will find that it is registrated for yahoo! inc.
It is part of their content delivery network, making pages load faster (hopefully) and distributing the load of the requests over multiple servers. I would guess that yimg stands for yahoo images. So any kind of multimedia content is probably going to be coming from there.
The reason that yimg.com is often refered to as malicious is based on fear and misinformation basically:
1)it seems to often be the slowest link, so many people notice it loading and slowing down their web experience when they didn't even request it opened, they worry it might eb malware.
2) It is a multimedia furnisher, therefore often uses flash, people see popups requesting yimg.com to store flash objects on their machine and don't know what it is and worry about the worst case.
They assume the worst and go to google. In comes the guy that wants to make a profit:
Lots of people search for something as a virus causes, let's call them dubious, programs and resellers to push their sales by offering themselves as removal tool on dedicated websites or removal pages. So now there are plenty of people worried this might be a virus and plenty of people eager to make you believe that it is a virus so you will buy their product.
At this point it becomes a self feeding process. Anyone coming online with "yimg.com" problems only gets virus-hits on google, then contributes by saying it's a virus, which causes the google hits to yield even more virus hits and even the people that are genuinely trying to help you (eg for free on forums) get dragged in because at this point it is reallly hard to find a single source saying this is not malware.