Tiggzi builder doesn't use WebKit, the browser uses it. The app that you build in Tiggzi simply runs in the browser. WebKit would cover Chrome/Safari. In which browser are you testing?
I think you should first test your code in a regular HTML page and see if it works.
I'm testing in Chrome. So i tested the code in a regular HTML page, with the Chrome Browser, and it worked perfectly.
This works:
code
$('img[dsid="pic"]').css('border-radius', '25%');
/code
That helped knowing i had the script correct. I moved the javascript to be loaded with the page, and it works fine now.