tell application "Safari"
do JavaScript "onCreateSubmit('Next')" in document 1
end tell
As you see, when you click the image, the function “onCreateSubmit” is called. As it is a function available to the document, you can invoke it yourself from your “do JavaScript” command…
http://scriptbuilders.net/category.php?search=sfri
If you are interested, this one includes a open-source script called “sfri Library”, which includes a script object called “Formularios”, which can teach you to manipulate any form element in a web page. There is also documentation and examples.
Basically, answering your question, for radio/checkbox elements, you set their “checked” property to “true” or “false”. For selects (dropdown menu, list-box), you set its “selectedIndex” property (eg, number “0” is the first item in the list).
Appart from this, any website talking about javascript is OK. I like this one: http://www.devguru.com/Technologies/ecmascript/quickref/javascript_index.html