You can find a letter format that you can use requesting permission to use images that you found on the net. I have also included links to some free resources on the last page of the presentation which I shared on SlideShare.
On a personal note, I like it that I am able to include what I have shared on my GreenbeingNancy blog to complete my course project. So, never let others tell you that what you are doing is not useful if you think it is meaningful. You never know where the future will lead you to when you are willing to share what you know.
If you have a problem going through the presentation, following are sites you can go to for free images. Be sure to read the terms and conditions before doing so:
http://www.clipartbest.com/
http://openclipart.org/
http://www.clker.com/
Even if these are free images, write a comment to thank the person who shared their work. I think that doing so will motivate them to continue in their effort to create and you may even like their responses should you hear from them.
So, a big thank you to all who have allowed me to use your work in my project.