Some images can be used without asking for permission from the copyright holder. In most cases you will not be allowed to alter images or use them for commercial purposes. Always strictly follow websites' terms of use & individual licenses applied to images. Do not forget to acknowledge the copyright holder. Below you will find a list of most popular image banks:
Creative Commons (CC) is a non-profit organisation aiming to help creators retain copyright while allowing others to reuse and share the creators' work. The organisation has released several copyright licenses, known as Creative Commons licenses, free of charge to the public. The copyright holder can apply a CC-license that will define how the work can be used and shared. You as a user will then be able to reuse the work without asking for permission.
Video: Creative Commons (3:04)
If you would like to apply a CC-license to your own work, you may find more information on the Creative Commons website.
Open Access (OA) is a movement focused primarily on the research literature. By applying an open license to a paper, barriers to copying or reuse can be reduced or removed completely. When compared to traditional library subscription model, research information marked as OA is available to readers freely. In the Open Access Guide you will find open resources that you can use in your research and teaching.