Photoshop has a built in automated image sizing program.
- Select File > Scripts > Image Processor...
- Check “Select Folder...” and point to where your files are.
- Check “Save in Same Location”. The process will create a JPG folder inside of the one that you just pointed to for the new, smaller files.
- Check “Save as JPG”. Set quality to 10 (the maximum is 12; the lower you go, the smaller the file size, but the worse the compression on the image).
- Check “Resize to fit”. Enter 800px for both width and height. This will ensure the longest edge is only 800 pixels, just fine for web. Smaller is fine, however.
- Check “Convert Profile to sRGB”. It’s safe to assume the majority of internet browsers out there are incapable of reading anything besides the sRGB colorspace, so this is a best bet if you want your colors to translate correctly from screen to screen.
- Hit Run. The script will open, resize and save a copy of those files you pointed to.