Let's assume we want to mirror twitter icon png.
First, place two rules at 0:0 point it will be convenient to align mirror objects around them. Place your graphic :
Place rectangle under the graphic make it the same size as graphic, deselect Fills and Borders check boxes, right click on rect and select Use as Mask:
Select graphic and rect and click Create Symbol icon in the Tools panel, this will make possible to change scale of your original graphic and its copy in the same time:
Duplicate sybmol by pressing Ctrl+D, flip it horizontal and place against your original graphic:
Now, you can select on of the graphic inside symbol and move it right or left, the copy will move too:
If you want to change the size of graphics select symbol and press Scale icon:
Scaling your graphic will move your graphics from original point:
But you can adjust its positions by dragging them and anchor to the rulers:
That's my approach to solve your issue.
You can download sketch file with mirror graphics.