All sprites are rectangular (or square) images. If you have a character in the ImageSprite that has a transparent background; the sprites 'boundary' is not the character, but the rectangular part.
Here is an illustration of two sprites and a 'collision boundary' problem. Both images have a rectangular size but the cirle and arc are smaller than the sprite dimensions. The image shows the boundaries of the sprite. These are the 'surfaces' which determine where a collision occurs; not the colored image part of the sprite.
The red and the green ImageSprites are actually rectangular objects . They appear round because the background of the sprite is set to transparent).
The ball will hit the red sprite before it can impact the green sprite in this example.
You probably need to crop your zombie image (which you did not share with us 
) to as close to the picture of the image is possible and decrease the sprite rectangular dimensions.
...so decrease the size of your 'collision' block and crop the character block so that it boundaries are at the edges of the sprites.