1. Copy these Images in gimp in two layers for that you have open one image in gimp and open other image as new layer .
3. Now in the third step make layer visible again by clicking the eye icon , no you have to make a layer mask by right clicking the layer select Add layer a mask then select the white color full opacity and then click ok thiswill add a mask to the layer . make sure you section is not lost .
4 . now in last step select the brush tool and now make sure the color of the brush is black and start coloring the selected area , and you will see that the portion of the below layer will be visible now and paint the whole selected area now will get some like this . Make sure that is whole process the selection is not lost .
5. Now you will see that there is some contrast in both the layer . you can minimize this effect by applying blur effect for this go to filters->blur->gaussian blur and set the value to 4 px and apply the effect . and in end you will get the final result