Is it illegal to download pictures from the internet for personal use?

In most cases, yes. However, some copyrighted materials are placed on the Internet for the specific purpose of being used by others. Check the web site for its policy on using the pictures. Be aware that a large amount of the content of the Internet has been stolen from the rightful copyright owner. Downloading stolen material may involve you in legal proceedings against the thieves.

It depends what kind of license is on the pictures. Some pictures have creative commons licensing that allows personal, non-commercial use with attribution. Look at the source and at any license. If a picture is not marked nearby as creative commons, then it probably is not legal to use it. To answer this question about any given photo, a lawyer would need to see the photo and its source.