Use a free photo editing app from the app store, i'm sure google/apple or even adobe.
Adobe Photoshop Express is available on Android, IOS (iPad and iPhone) and even Windows Phone Mobile, links and how to resize:
https://helpx.adobe.com/photoshop/using/photoshop-express-help.html
When the large MB images are uploaded, they're resized and shrunk in file size on the server so to speed download time for others.
If your wifi or 3g/4g connection is patchy, then if it drops during upload, then it will fail.
Resizing to 1280 max width or height (other side scales proportionally) and the file size is reduced, it will upload quicker and save data for those on low data plans.