Use 3rd party JS libraries (including JQuery), call Javascript functions, access object properties and methods. Implement game objects, and algorithms in Javasc...
Yes, you're right, that's the issue.
If you have a callback function like image.onload and want Construct to know when it happens, you can call a Construct function (Function plugin) right from javascript. Read here: scirra.com/manual/149/function («Javascript integration»)