imageCrop[img_] :=
DynamicModule[
{a = {10, 10}, b = ImageDimensions[img] - {10, 10}},
Graphics[
{Raster[ImageData[img, "Real", DataReversed -> True]],
{EdgeForm[Black], FaceForm[Opacity[0.25]],
EventHandler[Dynamic[Rectangle[a, b]],
"MouseClicked" :>
If[CurrentValue["MouseClickCount"] == 2,
Print[ImageTrim[img, Round[{a, b}]]]]]},
Locator[Dynamic[a]], Locator[Dynamic[b]]},
PlotRangePadding -> 0]]