How can I export several images with same size?

Usually selecting images layers with format name like image.png > File>Generate>Image assets allowed me to generate exactly same size of images but for some reasons, some images are not what I expected and not sure the reason and how to solve this problem.

I need 1000x1000 square images but as thumbnail shows images are already rectangle which tells me there images are not 1000x1000 square images.

What I did was to create 1000x1000 background layer then put image layer on it. Next was to group them as 1 layer and named on it as 0000.png.