What you have to do is post the image, then right click on it and choose copy image address.

the top link below is what you get, (it has the steemit url, followed by a slash, followed by two numbers. In this case 185x247. theyre the width and height. You can change them in the link and it will produce the same image in the altered dimensions.

Or you can also just put the following in front of your image link https://image1.steemit.com/111x222 where 111 and 222 are the width and height you want in pixels. but its hard to know what height and width you want until you see it posted

https://img1.steemit.com/185x247/https://www.steemimg.com/images/2016/09/27/follow8f3ef.jpg
https://img1.steemit.com/92x123/https://www.steemimg.com/images/2016/09/27/follow8f3ef.jpg
https://img1.steemit.com/46x62/https://www.steemimg.com/images/2016/09/27/follow8f3ef.jpg
https://img1.steemit.com/23x31/https://www.steemimg.com/images/2016/09/27/follow8f3ef.jpg