When adding borders, I've read that iWeb creates 4 copies of that object/image which may really slow down your site.
What I like to do is copy the image with the border (or even an image with a reflection) out of iWeb and into photoshop (with a transparent background). Then you can compress it a bit, (image>image size) and then save it as a .png (jpegs don't seem to work properly for me).
Now copy the image back into iWeb and you should reduce the load time of your site quite considerably!
I've done it on my website, and if you look closely at the navigation bar you can see that it is slightly 'jagged' at the edges due to the compression but I think that it looks fine from a normal viewing distance. http://www.JailbreakZeit.com
This also works great for reflections on images and other effects done by iWeb.
I hope this helps!