No. It is not legal to use someone else’s art or photography without their permission. If you are making something just for yourself and not showing it publicly, this “use” will never be discovered. However, it’s much better to get into the practice of asking permission if you are going to use someone else’s art in your own. If it was your art that was being used by someone else, you would appreciate the opportunity to say yes or no, and infringement of copyright can lead to legal proceedings so needs to be taken seriously.
There are lots of copyright-free images available in books and online. It’s important to find the actual wording, 'copyright-free' on the content so that you can use it without risk. Dover brand books publishes old engravings that are all copyright free. There are also many old books still in circulation or at used bookstores that can be good sources of imagery or text. However, you must be careful about the content and check if it is still under copyright. As a general rule, in the US, for works created after January 1, 1978, copyright protection lasts for the life of the author plus an additional 70 years. Regulations may vary from country to country and the onus is on each artist to check the relevant regulations.
Another grey area is around changing someone’s art or photography to avoid copyright infringement. It’s challenging to decide what is or is not enough change to make someone’s art no longer theirs. A good rule of thumb is that the art has to be changed so much that it would not in any way be related or compared to the original copyrighted material. This constitutes a significant change. Again, to be on the safe side, always ask permission or purchase the rights.