Comment 43 for bug 64147

I have a temporary suggestion. Simply download the Usplash source file from here:
https://launchpad.net/ubuntu/+source/usplash-theme-ubuntu/0.19
for example. But before building it, use the gimp to squash the widescreen image that's included in the folder and replace all (or at least the resolution you're using) by scaling without maintaining aspect ratio. Your resulting image should be distorted opposite to your current boot splash. Don't worry, that's what you want. Before saving each, select [image] menu [Mode], [Indexed]. Except gimps defaults and be sure to save as proper format and exact name of the file you intend to replace. (I leave the original file there so I make certain to get them all, and have to confirm overwriting). Then make it, and use startup manager to replace your current usplash with the .so you just created.

BTW: the lazy answer is to use startup manager to set splash resolution to the square screen closest to your screen height. The sides will be black bars like on a ws tv with square pic, but you won't notice 'cuse the Usplash is black anyhow.