Usually the way I include images is like this:

Then the only things you need to change are the link and the width, which is in pixels. Usually I just don't bother including the width="100" or whatever, and then it will either display the image at full resolution, or if the image is too large it will resize it to the width of the page. If you just directly drag or upload an image through the steemit.com site, you will see that the link looks kind of different than what I put, which is fine, I just like this way better, and I prefer to wrap the img statement in the 'center' tags to make it centered. You can also read more about this here: https://www.w3schools.com/tags/tag_img.asp