Figuring out if you need copyright permission to use an image can be complicated. Be sure to look for copyright information about the specific image you want to use from the source where you found it. On websites or in databases, there may be a general copyright policy for the entire site; look for links labeled "Copyright" or "Use and Permissions" or "Rights and Reproductions".
In general, you will want to be more careful if you are planning to put the image in a publication or on a website that anyone can access.
Here are some copyright resources that may be helpful in guiding you: