Why does my image look squished/stretched?
Momento360 currently assumes all source images are fully spherical images, so cylindrical panoramas get distorted. You can correct for some of it by padding the dimensions of the image with blank pixels to get to a 2:1 pixel ratio.
For example, here's a Collection with two images — before and after it has been padded to correct the distortion.
In the original, you'll notice the "pinched" look at the bottom of this image:
(you'll need to move the camera up to see the rest of the image)
Here's the same image that's padded with white pixels (added blank/white pixels to the top and bottom to get it to 4096 x 2048):
If you switch back and forth between the two via the link above you can see the difference.
Any image editing program (Photoshop, Pixelmator, Snapseed, etc.) will allow you to add blank pixels to the image. It's a little like printing on a piece of paper that's bigger than the image, in this case you want that blank space at the top or bottom of the image.
Here's an online program and walkthrough that should show you how: http://curiouslight.com/change-canvas-size-pixlr/