If you do not have Infranview installed, download it from the Infranview official web site and download.Use default options - just follow the prompts.

When you have Infranview installed, find the folder with your pictures.

Right-click on the image, select the 'Open with' option and left-click on the 'Infranview'

Click on the 'File' menu and select the 'Batch Conversion/Rename...' option.

Select the batch conversion, make sure that the output format is set to JPG, select the 'Use advanced options', and click on the 'Advanced' button.

Click on the 'Resize' option, then 'Set one or both sides to' and then on pixels. Set the size. For the web purposes, it can be 640x640, or 1024x1024. You can use different settings too, just do not set the resolution too high. Click on the 'OK' button.

Select the 'Output directory' where resized images will be stored, click on the 'Add all' button and then on the 'Start Batch'. After a few moments, or minutes depending on the number of images and the speed of the computer, all selected images will be resized and good to go.