Importing Graphics Rpg Maker XP/VX?

I was trying to make an event with a shadowy fish i made and when i imported the image, when i made the event, I selected the fish graphic I imported and when l tried to select the square for the fish, it made it so that it selected a rectangle, and not a square that went perfectly on the fish. how do l fix it so that it makes the square selection go on the fish right? (i use xp if that helps)