I saw this on VHS when I was a kid. It's a '70s-'80s film about shapeshifting, sea cockroach-looking aliens that were invading earth. The plot centered around a high school senior/college boy who had a crush on this cover girl, and one of these aliens stuck itself to one of her posters and transformed into her. I believe they dated and then maybe, just maybe the girl alien helped him survive? Or beat the other aliens? Or did she turn on him too?
I remember the main character took her out on a date to one of his schoolmate's house parties, and everyone was jealous of him. It was a proud moment for the young lad, for some time anyway.