Click the Edit button from the top toolbar (It looks like a Pen icon.). Now that the photo is open in Preview, it’s time to resize it. RELATED: How to Change the Default Application for a File Type in Mac OS X
If Preview is not set as the default image viewer, right-click, and choose the Open With > Preview (We recommend that you take some time to set Preview as the default image viewer, as it will make your life easier.). Then, simply double-click the file to open it in Preview. First, open the Finder app and locate the image (or images) that you want to resize. Using the Preview app, you can do this for one image at a time or for multiple images at once.
If a website or an application has an upper limit for a photo file size, first, try reducing the resolution. The fastest way to reduce the photo size is by reducing the photo resolution. How to Reduce Photo Size by Reducing Resolution
