Yeah, I wish more site designers would think of this...it's so easy...Apple even takes care of the rounded corners and gloss effect, all you have to do it upload an image. I did it for a blog in about five minutes from start to end.
In the meantime, zooming in before adding the bookmark should let you at least choose which part of the page you want to use. In other words, the icon will just use whatever is on your screen, so try zooming in on the site logo or some other interesting bit before hitting the + sign.
I had the same problem. I hate the screenshot icon for sites with no icon for the iPad/iPhone/iPod touch.
There's an app in the app store which called my clips which does exactly that. The free version got everything to do what we ask. The pro version can make the same links for SMS and calls for what i know but. Didn't try the pro one coz I'm cheap :-)
Well... Although the program is not as easy and comftarble is I would imagine it does the job.
Just look for myclips in the app store