When you use an image that you created, the answer is easy. You decide on how you want to credit yourself. Do you want to place you image in the public domain or do you want to protect it with a copyright notice. You decide.
When you use an image you found on another site, it's not always so easy. The first thing I do is look to see if there is a copyright notice on that site. If there is, I don't use the image.
Some sites, especially those that offer free use of images, will tell you the method of attribution that they require. I look for that info and I place the attribution notice under the image in the exact way that is requested.
Some site require an attribution along with a link to the page where you took the image from. Others will just require the name of the author. The important thing is that you need to look for that info and honor their request.