Hi Yakir,
I missed some part of the question, sorry. You were quite close with your guessing. The correct implementation is @on(load, "newimage=true").
Note: you have to add an additional empty string argument, because the first non-keyword argument is always placed between the opening and closing bracket of the element. (The @on macro produces a regular argument, try it out at the REPL.)
So your quasar element would look like:
quasar(:img, "", src=:imageurl, spinner__color="white", @on(load, "finished_loading=true"))