img=;
imgResized=ImageResize[img,Scaled[0.25]];
SetOptions[EvaluationNotebook[],ImageSizeMultipliers->{1,1}];
imgSize=ImageDimensions[img];
imgResizedSize=ImageDimensions[imgResized];
centerPoint={((imgResizedSize[[1]])/2),((imgResizedSize[[2]])/2)};
rectangle[imgSize_,width_,relCenter_]:=Module[{centerPoint},
centerPoint={imgSize[[1]]*relCenter[[1]],imgSize[[2]]*relCenter[[2]]};
Rectangle[centerPoint-(width/2),centerPoint+(width/2)]
];
width=25;

Manipulate[
Row[{
LocatorPane[Dynamic[p],
Show[
imgResized,
Graphics[{EdgeForm[Directive[Thick,Green]],FaceForm[None],Dynamic@rectangle[imgResizedSize,width,(p/imgResizedSize)]}],
Graphics[Locator[Dynamic[p],Appearance->Medium,Background->Red]]
],Appearance->None
],
Show[
Block[{σI},
σI=(Sqrt[(σ^2)-(0.5^2)]);
ImageConvolve[img,GaussianMatrix[{Ceiling[6*σI]+1,σI}]]
],
Graphics[{EdgeForm[Directive[Thick,Green]],FaceForm[None],Dynamic@rectangle[imgSize,width*(σ/0.5),(p/imgResizedSize)]}]
]
}]
,{σ,0.5,10},{{p,centerPoint},None}]