Using the default object properties for a Picture Variable the On Clicked form event does not trigger immediately when the mouse button goes down. It waits for the click to be completed and the mouse button to come up.
This is because Picture Variables are "Draggable" and "Droppable" by default. These properties make it so that the On Clicked event does not fire immediately. Disable these if you want On Clicked to fire as soon as the mouse button goes down in a picture variable (similar to the functionality on an invisible button object).