Well, first, make sure it is free. then you can right click on it, do a copy image then paste it in your editor. Example this image:

Will look like this as code when pasted into your editor:
https://steemitimages.com/DQmXmdadCYFMH2MEcHc92CYHHhVCUyCzqEUw4V3GXxWUosF/bar1.PNG

There are many great post on using the Steemit editor. A couple favorites of mine are this one buy @thinkagain and this one by @itadakimasu there are tons of others. I usually enter a search like... "how to center a picture in steemit markdown editor?
I hope this helps..Thanks for the comment! CARRY ON!