Where do you find images?
Google has an image search site Google Images, and there are many others; the Creative Commons allows you to launch searches of many sites. Whichever source you use, select an image that allows you to use it.
Yes, you heard (read) right!
Just because an image is on the web doesn't necessarily make it yours even though it is easy to save and copy. If an owner of an image does not allow use, you can be in breach of copyright when you do this. In any case, to cite your work properly the more you know about the image you are using the better. Great news! - there are many sources that allow you to use their images, or use them and attribute the creator.
All you need to do is check that your search results allow you to use the images freely. For instance on Google Images go to the Advanced Search feature (currently in the bottom right corner of the screen under 'Settings'. The last option on the page is :
Click and select your preferred choice, eg free to use, share or modify is a good option.
Cite your image as you would any other type of work. In the same way you use citations to acknowledge other peoples' ideas you find in books or journal articles, cite the source of any images you use.
Here is a guide to using images fairly. It even has an example case.