How can you be sure you’re not infringing on copyrights of other artists? Sadly, there’s no way of knowing unless vindicated in court. No one wants to have to go to court, so seeking legal advice is a smart choice.

Under the copyright law in the U.S., infringement is “substantial similarity”, meaning an average viewer would notice that copyrightable authorship was taken from the original artwork.
Regarding design and pictorial elements, a copyright covers expression, not ideas or underlying concepts. Meaning that you can depict concepts used by other artists, but you must convey original artistic decisions made in expressing the concepts.

Using copyrighted images as references

To use other people’s images as references, the law requires that you transform the image. This word is interpreted differently by courts around the nation, so take it with a grain of salt.

Some courts may refer to transform as changing the purpose behind the an image’s use. Others may refer to it as physically changing the image.

Even if you’re drawing straight from an image solely for practice, it’s considered infringement because it’s a derivative work. It doesn’t matter what your intentions are, creating derivative works, copies, or distributing the copies or derivatives is the exclusive right to the artist of the original.
Get permission to use images and make sure you have proof of permission. Licenses from copyright owners, usually available for a fee, allow you to use images and provide proof to keep you out of trouble.

Using images from the public domain

Images in the public domain don’t have copyright owners and are safe to use without permission. There are multiple websites that provide public domain images including Wikimedia Commons and Unsplash.

If you’re specifically looking for images of artwork, The Metropolitan Museum of Art has hundreds of thousands public domain images of professionally documented artwork.

Using life references

The safest way to avoid infringement is using life references, rather than images. For example, use a live model to reference the human figure and go outdoors to reference nature. This is not the case if your life reference is an object, such as a painting or sculpture, with owned copyrights and you don’t have permission from the owner.