This is an odd little tip...not sure where I found this out, but I use it a fair bit...if you have an image which you're not sure will be available, add this to the img tag: OnError="this.src='/images/shim.gif';this.width='1';this.height='1'" (where images/shim.gif is an image which will ALWAYS be available - if not you'll get an odd error...).
Anyway, prevents the ugly broken image thing...