Pixel Collision

Hi,

At the moment my tiles are all 16x16 pixels and they take up all this space. Now, I want to add tiles that don’t, think half a tile, thus top half is transparent, bottom half is filled. So, when a collision occurs, what is best way of handling this type of tile so player moves down half the tile. Would you need to detect this type of tile and allow player to move down 8 more pixels? Would I need to use the alpha channel of the tile or something?

Thanks