Images are a great way to capture the reader's attention. But please be careful where you get them from.
If you just got your image "from Google", or some random website, chances are high that you can not use it. Under the Berne Convention, copyright applies even if not explicitely stated.
More specifically, we have to assume that photos under non-commercial licenses (e.g. the popular CC-BY-NC Creative Commons license) can not be used on Steemit because you are earning for your post, hence, commercial use.
If you do not abide by licenses, chances are you can get sued, and no one wants that.
It is best to pick public domain images. Public domain content can be used for any purpose you want, and it is not required to give attribution (credit) to the author.
Here are some great sources for public domain photos:
https://pixabay.com/
https://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources
http://www.public-domain-photos.com/
If you know of other good sources, please add them in the comments!