ImageGarageInstagram still treats 1080 pixels on the long edge as the useful square feed size. Sending a 12 megapixel original just makes the app compress it for you, often worse than a deliberate resize.
Square post
Open Resize Image. Uncheck Keep aspect ratio, set width and height to 1080, then click Resize. That stretches the photo to a square. If stretching looks wrong, keep aspect ratio on, set width to 1080, download, then crop to 1:1 so people stay in frame.
Portrait and landscape
A 4:5 portrait is typically 1080×1350. A 1.91:1 landscape is typically 1080×566. Use pixel mode with aspect lock, set the 1080 side, and let the other dimension follow. Then crop if Instagram’s in-app crop still clips the edges you care about.
After resize
If the file is still huge, run it through Compress at 80–85% JPG quality. ImageGarage processes the file in your browser; it is not uploaded to our servers.