aerial picture

[Kenton Williams]

If you're having trouble georeferencing your image, you could try some free / open software like Landserf or Hypercube.  Landserf is probably the easiest (www.landserf.org) and all you'd need is three ground-units coordinate pairs for three
(preferably well-distributed throughout the image) image-coordinate locations in the image for a 1st order (linear) transformation.  The appropriate world file would be generated for the resulting transformed image.    

Is your photo a digitized aerial photograph or an ortho-corrected photo (orthophoto)?  If it's not ortho-corrected, it's best to perform some sort of corrective transformation on it before using it in a mapping application.

An aerial photograph is NOT a map due to displacements caused by lens distortion, terrain relief variation, and the effects of a projective lens system (non-orthographic).  If the image is an orthophoto, these effects have been removed as best as
possible and can be treated generally as a map.  You can transform/warp a non-orthocorrected image (using Landserf or others) into place using a 2nd order (quadratic) polynomial warp, which is sufficient to adequately correct for most
displacements/distortions mentioned above.  The drawback to the second order polynomial warp is that you'll need at least six control points instead of three ((N+1)*(N+2)/2 control points for an N-ordered polynomial transformation).  A cubic (3rd
order) polynomial warp can correct many more complex distortions, but it can also introduce some rather unpleasant new ones, so I rarely use a 3rd order warp unless I have a large number of very well-distributed control points.

If your image is already rectified (is an orthophoto), you should be able to just create a world file rather than transforming the image.  If I'm not mistaken, that's what the QGIS georeferencer is attempting to do.  You will just need to figure out
the coordinates of two corners and then compute the pixel size based upon their differences along each axis.  You can actually create world files in a text editor!

Kenton W.