What you DON'T hear a lot about is the use of logos, sayings, music, textures, and other materials available on the Internet.
Just because you can copy it, does not mean it is not copyrighted.
Pretty much anything you recognize as a name brand has every bit of their material copyrighted. That means it is illegal for you to use their slogans, icons, logos, etc. in your creations. No, they won't give you permission, either.
Music bands have their logos and artwork copyrighted, as well as their music. Again, you cannot use their stuff in your creations.
Museum sites which display artwork on their websites have copyrighted the photos and other materials. You cannot use them in your creations.
Most scrapbooking sites will not allow you to use their materials, purchased or otherwise, in virtual world creations. Check the site's copyright, EULA, or IP rights information.
Some photographic sites will allow you to use their materials in your creations - but will NOT allow you to sell their work as textures or artwork in virtual worlds. Again, check the information on the website.
Famous paintings cannot be used. User-generated art may be used, depending on their copyright provisions. Always check before use.
If you want to use a person's photo off Flickr or another site - always ask for permission before using. Same goes for photos in blogs and other such sites.
If you self-generate something anywhere on the Internet - as in, from scratch - it becomes copyrighted from the day you publish it.
You may think you won't get caught, but that right there is a fallacy. Not only is it possible that the copyright owner could file a DMCA against you - those who are hosting you and your creations may "catch" you and demand you stop selling the items.
A grid has to stay neutral until approached by a copyright holder. Their staff may in fact recognize the illegal use of something, but by law, they have to wait for a DMCA takedown notice to be filed.
Landowners within a grid do not have to remain neutral, and here is where some good can be done. If you rent or sell land to businesses, take the time to "walk your sims" now and then (and if your holdings are too large, have your estate managers do it). Make it clear that you will not tolerate the misuse of materials copyrighted by others. Take action, should you find someone who is clearly violating someone else's copyright - even if the other person is represented on the greater Internet, rather than on your grid.
You might even go so far as to attempt to educate others, though common sense would tell most people that "recognized entities" such as a band, brand name, or otherwise, will not allow you to legally use their stuff.
Sorry, you can't justify the use by "but we're giving them free advertising". I'm sure if they wanted to advertise on a grid, they'd have a presence there, and be doing it themselves.
It's simply not right to try to profit, in any way, shape, or form, off another's copyrighted work.