Scroll down and click “Upload Now” and select the image from your photo library (if you’re on a phone/tablet) or from your computer.
From the “resize images to” drop down, choose “Compact” and then click Submit.
Once it finishes resizing your image, click on “Download”.
It will then show you the resized image. If you’re on a phone or tablet, you can press on it and get the option to save it back to your photo library. If you’re on a computer, you can right-click and save it.
Then you can return to the Forum and try uploading again, as this newly-saved image will now be small enough.