Click HERE to download a free Photoshop CC Action that will help you resize, sharpen and save images for WEB and for FB. You can use this to prep your images for IG as well but IG would only accept JPEGS so save your files as JPEG instead (select Preset as JPEG). For WEB and for FB I would definitely recommend saving your images as PNG24. You can reduce the size if you wish as this keeps changing all the time. Check the sample below: