One must complete due diligence before using an image. Sometimes it is as simple as seeing if the image falls under the Public Domain or under a Creative Commons (CC) License. If the image is not in the Public Domain you will need to look at the Fair Use Act to decide if the use is allowable. Below are some questions that may help you determine the legal use of an image. Although, keep in mind this is not a legal answer, only a tool to help you determine the allowable use of an image.
1. Is the work covered by copyright?
2. Is your intended use permitted?
3. Does your intended use fall under Fair Use?
4. Can you get permission?