That is some *beautiful* sprite work in your art, especially for a beginner. Unfortunately I couldn't play the game (web version)--I clicked "run game" and none of the inputs worked, so I can't rate gameplay.
Played the game, and I like the swapping-powerups idea, but it had the effect of making the non-powered character movement feel sluggish and 'not fun', which made me not want to use the poison powerup that much.
Might I offer some pixel art advice? Use a consistent scale--make sure the sprites, background, and any other objects use the same pixel count per tile. It'll make your art look even better, first of all, and will also be more authentic. Shading is also important, as is learning what objects look like IRL to be able to better duplicate them digitally.