Welcome to steemit.

You should consider including a source for your images. Including a source not only gives credit to who made the images but shows you put time and effort into making your post. This may make people more likely to vote on it.

If images are Public Domain or CC0 you could just indicate that at the end of your post. All photos are Public Domain for example.

Finding the original source isn't necessary. A link to where you found it will do.