You can't resize images in 1.8, whether you type out the MyCode directly or use the editor to resize the image.
Examples:
http://community.mybb.com/thread-157427-...pid1092260
http://community.mybb.com/thread-157440.html
http://community.mybb.com/thread-157441.html
[img=250x300]http://chayagrossberg.weebly.com/uploads/4/9/4/5/4945992/9674878_orig.jpg[/img]
[img=500x100]http://news.distractify.com/wp-content/uploads/2014/01/new-userguide-image.jpg[/img]
[img=400x400]http://www.cleanwateraction.org/files/images/ca/Front%20image_drinking-water.jpg[/img]
Examples:
http://community.mybb.com/thread-157427-...pid1092260
http://community.mybb.com/thread-157440.html
http://community.mybb.com/thread-157441.html