Public domain images can be a great resource for designers of all types. You just have to make sure you actually understand what this means. Just because an image is publicly available, does not mean it is public domain (sorry, Google search masters).
Public domain is an intellectual property designation that applies to images whose copyrights have either expired or been purposely relinquished by the author. As such, they are free for all designers to use and modify.
Of course, it is still not a good idea to make a public domain image the basis or main component of any design that hopes to be trademarked, like a logo (since anyone can use the image, it is unlikely that such a design would qualify for trademark protection). They are better suited for functioning as a lesser component in the scheme of a larger design; for example, an icon on a website or way-finding system, like the ones above which were created and released into the public domain by AIGA and the Department of Transportation.
Sometimes finding suitable public domain images can require a bit of searching. However, if you’re looking for a common symbol (i.e. fleurs de lis, the recycling sign, etc.), finding a public domain version should be pretty darn easy. Wikimedia is one reliable place to look.
Below we’ve put together a list of common symbols, all of which we were easily able to find versions of in the public domain. There’s no excuse for copying another designer’s work when you can simply find a copyright-free version and modify it to suit your needs.
Male and Female Bathroom Symbol
No Smoking Sign
Star of David
Scales of Justice
This is of course only a very small smattering of symbols commonly available in the public domain; no doubt you can think of many more!