。
I can't forget how my mother looked.
Literal
Mother [gen-の] visage [obj-を] cannot-forget.
面影 is a poetic/evocative word for a remembered image or a trace of something — often used for the visual memory of someone gone. It's not just '顔' (face); it carries the softness of 'the way they looked in my memory.' 忘れられない is the potential negative of 忘れる. Subject (the rememberer) is omitted — plainly the speaker.