This is a short guide on how to choose the right images to use for a puzzle. There are some restrictions on images that can be used. Most images found online are copyrighted and may or may not allow being used on another web site like this one. Also, images that have questionable content will not be permitted.
If using an image that you found on a different website, then make sure to copy the link to a page that has the copyright information. This is usually required, in fact it's the only way I'll know that the image can be used. If it's an image that you made and don't have a link to it, then just make sure you state that in the description. There is no need to put your name in, if you don't want to.
Learn more about some of the creative common licenses at their website: Creative Commons
Good websites that host images
The following are some websites that usually have some good images with copyrights that allow using the image elsewhere.