Remember that when using images in your work you must identify where you got the image from and reference it correctly according to the referencing style you are using.  If your work is being published or will be publicly available (e.g. in your thesis) you will need to get permission from the owner of the image to use it in your work. Some of the links below link to images that are able to be re-used without needing permission from the owner of the images but you still must acknowledge where you got the images from. 

Explore these sites to find copyright-free images for your work. You don't need to ask permission but there are some that you have to attribute to the creator.