Rodney, here is another one this will work out the scale needed to fill the pic box in either direction and apply proportional scaling to suit. Then centres too.
tell application "QuarkXPress"
set CurBox to object reference of current box
set bounds of image 1 of CurBox to exact fit
copy (coerce ((scale of image 1 of CurBox) as percent point) to list) to {VertPercent, HorPercent}
set VertPercent to VertPercent as real
set HorPercent to HorPercent as real
if (VertPercent > HorPercent) then
set scale of image 1 of CurBox to {VertPercent, VertPercent}
else
set scale of image 1 of CurBox to {HorPercent, HorPercent}
end if
set bounds of image 1 of CurBox to centered
end tell