How to batch resize all images in a folder (including subfolders)?

I have huge, 12GB, gallery on the server, full of images in various subfolders. Those files are too big and not used in full resolution. I need to resize all the images down to 820px wide (keeping proportions). So my question is - how can I create some kind of crawling script which would resize all images bigger then 820px and save them back overwriting original file? Hope you can help me :-) Thank you in advance.