You will need to either use microsoft picture or similar program already on your computer, or a 3rd party program.
If you right click on the picture on your computer you should get a option that says Open with, if so click on that and see what programs you have, try each one. Once open look for the edit tab and resizing option..
This is just a couple of ways, Im sure others will chime in..
You can also just drag and drop your photos, but sometimes if to large they will be huge on here!