Yeah its pretty simple to mirror something. It can be done in Paint but its is much simpler to do in Photoshop.

Paint
1. Open the image you want to mirror.
2. Open the drop down "Image"
3. Flip/Rotate image either Horozotially to change from right to left
4. Cut/Crop the portion of the picture you want to use and copy the image.
5. Open the image you want to add the Croped picture you just copied ( might want to have 2 windows of paint open for this step, one with your croped picture and one with the picture your are adding the croped picture to.
6. Paste the croped picture on top of the other and arrange it to where you want it.
7. (optional) blend/fade it in to seem natural to the other picture.

Far as doing this in Photoshop.... It is much simpler not to mention the final product looks much more professional. But paint can look good too if you go all the way down to the pixels to edit it.

This I did all in Paint cutting and croping images I collected from all over the place.



My signature was done in Photoshop....



You can tell the massive difference in final product quality. But if you do not have Photoshop and can not afford it paint doesnt look TOO bad when your done, it just takes more time to smooth out rough edges.