Q: I'm thinking of selling my homemade crafts online. But I use existing artwork in my products, and I don't know if that's legal. What do I need to know?

A: If you make and sell photo frames using advertisements from magazines, model airplanes from Coca-Cola cans or knitted jumpers that bear the logo of your favorite sports team, chances are you're infringing on someone else's intellectual property.

Tony Ford, co-founder of online artisan marketplace ArtFire, says infringement is bound to happen when people are making things they're passionate about. "At the early stage, these are people shifting from being hobbyists to businesspeople," he says. "They make things they love, but a lot of the time, what they love is the intellectual property of someone else."

There are a couple of potential minefields here: infringing on copyright, which covers original creative work like paintings or photographs, and stepping on trademarks, which cover words, phrases, symbols or designs like company logos.

With copyright, a good rule of thumb is to assume all work in the U.S. after 1923 is protected by copyright—anything before then is in the public domain, says Sergio Muñoz Sarmiento, a visual artist who also teaches art law at Fordham University.

Between 1923 and 1978, work either had to bear a copyright symbol or be registered with the copyright office in order to be copyrighted, he says. Since 1978, however, work no longer has to be registered or even bear a symbol to have copyright protection. "If you draw a design on a sheet of paper and post the image on Facebook, and I grab it and make T-shirts out of the design, I've infringed on your copyright," he says.

If you do use a copyrighted image, are you immediately in trouble? Daniel J. Brooks, a partner in the New York office of law firm Schnader Harrison Seagal & Lewis LLP, says the primary question in determining intellectual-property infringement is why a work was used. Using a copyrighted image for the purpose of parody, criticism or explanatory or educational purposes, for instance, could be protected by fair use.

The best thing to do before you create a piece that incorporates someone else's artwork is check if the work you're using is protected, advises Mr. Brooks. If you don't, and you receive a "cease and desist" letter that accuses you of violating copyright, he suggests you immediately stop selling your work.

You might also negotiate a license to use somebody else's images, but the fees can be steep for small companies, warns Mr. Sarmiento.

As for trademark infringement, such as an artist who, say, turns Jim Beam bottles into ashtrays, one test is whether customers are likely to be confused as to its source, or if customers could think the brand has allowed the use of its trademark. Another is whether the image or logo is used in a way that could tarnish the image of the brand.

In general, most corporations won't prosecute if their logo is incorporated into works of art, says Mr. Sarmiento, provided the art is sold in small quantities and the work doesn't specifically use the company's name to make money or tarnish the brand. So, for instance, incorporating a McDonald's logo into a painting of Times Square is most likely fine; selling T-shirts featuring the logo is probably not.

Apart from intellectual-property violations, Mr. Sarmiento says artists could also violate the right of publicity, which limits the public use of one's name, likeness or identity for commercial purposes. Although the right of publicity varies by state, there are certain broad principles it's good to keep in mind—the foremost being that people have the right to make money off their own name, image or legacy.

Ms. Chaudhuri is a staff reporter for Dow Jones Newswires in New York. She can be reached at saabira.chaudhuri@wsj.com.