I made this tutorial for newbies like you so that maybe you'll become a good thread poster or something else that will help the forum for contributing posts.
SMF Bulletin Board Code TUTORIAL For Newbies
And to resize it simply insert width and height.
[img PUT THE WIDTH AND HEIGHT HERE][/img]
[img width=200 height=150] PUT THE DIRECT LINK OF THE IMAGE HERE[/img]
Here's my other post about posting images and changing it size. You can also go here HELP.
Insert Image
Press the button
Example:[img width=200 height=150]http://i66.tinypic.com/10pvrjd.jpg[/img]
Note : Only use direct link of the images you want to post.