I use the very popular Irfanview.

1. Open the pic with irfanview.
2. Click Image menu.
3. Select Resize/Sample.
4. You'll see two edit fields labeled Width and Height on the left hand side. Whichever number is larger, change it to 100. Click OK.
5. Save the photo.
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"All my years in p*rn didn't quite prepare me for childbirth. I mistakenly thought all the stretching I did would make this easier."