You need the right software... photoshop or paintshop pro or summat...
Irfanview will do it and is free.
Once you install it, open one of the picture files with it & click Image & Resize/Resample.
Probably easiest to then click "Set New Size As A Percentage Of Original" then enter the % size you want, and save the file again
Open image, click on Image on the options at the top then click on image size (refer to top of pic), then in the options box, put in what size you need the photo (mm and resolution at bottom of pic)