Does the quality of fill dirt matter?
If you were pouring a slab over this -- and required decades of stability -- you might need to worry about your fill.  Organic matter and other debris that will break down over time is appropriate for topsoil but not for fill.
Your project is going to be less sensitive to settling.  You should compact your fill well as you place it.  It looks like you have way less than a cubic yard in your photo, so you can easily do it with a hand tool, like a tamping bar or a lawn roller (see equipment rental yards for the roller.)  Make more than one pass of fill-compact-more-fill in the deepest area.  A few inches at once should compact well with a small lawn roller and won't take all day!