I'm not sure if I've understood you correctly, but:
Grab the right-hand grey dot (the pointer will turn to a double arrow) and drag it out to leave a white space. click-n-drag around the original picture to select it. Copy, paste. Position it, then flip it as per above instructions. The position as required. Hope that helps, apologies if I've misinterpreted your question and muddied the waters.
ada.h You have the right idea I click on (Select All) but I have to grip the pic from the top left. I shrink it right down to the corner, but it still comes up large covering the page, so when I click on select make a dotted line around the pic, click copy and past I am missing the wing of the aircraft.
In Paint click on the tool select (looks like a dotted rectangle)
On the picture drag around the part that you want to mirror (it will become surrounded by a dotted rectangle)
put cursor inside this dotted rectangle and right click and then left click on 'Copy'
move your cursor to a blank part of page and right click and then left click on 'Paste'
the pasted part will be surrounded by dotted rectangle and you now go to 'Image' on dropdown menu click on 'Flip/Rotate' put your dot in the circle of your choice click on OK
You can move the position of this selection by putting the cursor (4 headed arrow) inside the dotted rectangle and dragging it so that it is 'nose to nose' with your original pic
Now move your cursor to a blank part outside the dotted rectangle and left click and the dotted rectangle will disappear and then you can click on 'File' and then click on 'save' and the changed picture will be saved.