Go to the Squoosh app. This is a tool provided by Google for the purpose of optimizing images for use on the internet. You can use this tool when uploading images to any platform, not just WSG.
Drag and drop an image onto the window of the Squoosh app. Your image will be displayed in a side-by-side presentation: on your left is the original image, and on the right is the optimized image. There is a sliding bar that you can move to reveal more of the left or more of the right image.
In the lower right panel, ensure that the Compression is set to MozJPEG or Browser JPEG (it should be by default).
Toggle the “Resize” button and the corresponding panel will open. All you need to do is change the appropriate dimension (if landscape, set the ‘Height’ to 1200, if portrait, set the ‘Width’ to 1200, if square, set either one to 1200).
Visually inspect the image. If the original was a high-resolution image, the optimized image may look a bit fuzzy, which is normal. If the image looks good, you may want to reduce the quality (move the slider to the left) a bit in order to reduce the file size (smaller files display faster).
Click the download button in the lower right corner and the optimized image will magically end up in your “Downloads” folder.
Upload the image to this (or any other) site.