animation outline

can someone please explain to me why sometimes when i add animation it takes out the non rectagle image from the file like i want to, and sometimes it takes the whole image
What is it that makes the difference
how do i make it so only the image gets piked out not the white behind it
ie:
whole image
////////////////
////////////////
ooo///////////
oooooo///////
oooooo///////
ooo///////////
////////////////
what i want
ooo
oooooo
oooooo
ooo
you know what i mean




ie:
whole image
////////////////
////////////////
ooo///////////
oooooo///////
oooooo///////
ooo///////////
////////////////
what i want
ooo
oooooo
oooooo
ooo
you know what i mean

