I want an action to be run (an image must be displayed) when the user presses the Enter key after writing a text in a text field. How can I do that? Unfortunately Courselab doesn't have a "onkeypressed" event.

Jquery or javascript
You might find a snippet like this useful http://www.syntacticsugr.com/23-javascript/sugr_cubes/81-globally-capture-key-press-events
This could be added as an initiating event by using a Javascript object to detect a specific key press.