Is a Muslim sinful if he accidentally looks at a forbidden image, or if he goes back to make sure the image is forbidden after having passed it?

There is no sin upon one who unintentionally looks at a forbidden image by mistake, based on the saying of Allah the Almighty: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended." And based on the saying of Allah the Almighty: "Our Lord, do not impose blame upon us if we forget or make a mistake." And Allah has responded: "I have done so." One must immediately avert their gaze from the forbidden image; otherwise, they would be sinful if they intentionally persisted or returned to look.