If you are taking the image off the web then you only need the web address of the image, usually ending in jpg though several other formats also work, then just add your width= to the img tag supplied with the UKP emoticons.
If it's something on your hard drive, you need to upload it somewhere first, and also if it is online but doesn't have a separate picture address. An advantage of Imgur is that you can add a Firefox browser extension that does it automatically. Right click for crop marks and right click again to upload, then paste the direct link into UKP inside the img tags (which is what I did on the kamagra thread a few minutes ago!)