Open you picture.
Select the crop tool. Choose manual. Select a crop region that is as close to square as you can. Click OK. This is the only way to get a square picture in picasa.
Then go to File menu and select "export picture to folder..." in there say "resize to 1920 pixels" and "Image quality maximum". This is the only way I can find to resize the image. Pick a destination folder and click OK.
Go find your file and upload it.
GK