Really love the beautiful spritework. The little animations like turning frames for your character and slight wobble on the moving enemies really brings it all together. The gallery is a very nice addition to give players a reason to try to clear the game multiple times. The background however felt a little empty, it could use a bit of texture. I would also like a different visual cue for when an enemy hits you.
I think that the holding the mouse button to fire is unnecessary. You could have the right mouse button toggle autofire, or add reasons for why players might not want to shoot.
In terms of balance, the game stops getting harder after a while and there's periods of no enemies appearing in some of the levels. Perhaps an option to increase starting difficulty can be added for players who want to focus on getting new highscores? I also suggest adding more enemies like that mutant doll that sit in the back, have a lot of health and periodically cause trouble, so that the player will have something to shoot at after clearing any immediate threats. You could also add some recolored enemies for later levels that just have more health. Jacky is definitely too strong and could probably use a decrease in max life and damage, shouldn't be taking down the mutant dolls in a single hit.
The sound design is very good. I like that everything makes a different noise and the ambient music track fits the action nicely. The shooting sounds, especially on the fast fire rate characters like Greg could be slightly more quiet.
Overall I really enjoyed the game. With some slight balance tweaks and quality of life changes, it can really be a game that players will want to fully clear and then play again just to get a higher score. Very nice work!