Try putting a ! before the file's name. For example, if the picture was called Person, call it !Person.
Wrong - correct idea, but wrong letter, please check your answer before posting it ;-)
@OP:
by default, Ace expects the spritesheet to contain eight spritesets of persons. To override this, there are two different symbols that need to be placed into the filename.
$ is the sign for single spritesheets, without it the selection box is for the size as if the sheet contains eight different sets of a sprite.
! overrides the displacement of the sprite, that is neccessary for door sprites or other structures: without !, the sprite is displayed a bit higher than the regular grid, to give the illusion that the person is walking in the grid position instead of its bottom line. For structures that need to fit to the grid like doors or chest, you add the ! to the filename.
For a single door-sprite, you would need to add both ! and $