I am imagining what my imagining behavior actually looks like. It is a colorful complex interconnected network of neurons firing.
To a lay person, that networked pattern of neurons firing would probably look similar to when I was imagining something else, let’s say a duck.
When I stop imagining the duck or the neurons firing, the picture of those things is no more, until imagined again. (Now the pictured duck never existed. The pictured neurons firing, did exist, but probably was vastly different than as pictured.)
Any behavior (including any mental behavior) exists at the moment that it occurs and not else when.