Hi Lisa,
Sorry for the tech-speak. Aspect ratio is the ratio between the width and height of the image. Example: a picture with pixel width of 400 versus height of 300 has a width to height aspect ratio of 4:3. That example fits the composite image you created for the header and gallery image, i.e. its width is 883 pixels and height is 671 pixels, or about 4:3 aspect ratio for width:height. That's why it displays as that ratio at 395 x 300. Getting it to 400 x 400 requires stretching the image using resizing software, which is why the ones that (I assume) you "fixed" look stretched.
So, long story short, your original image is the problem. If you want an image to display as 400 x 400 in the Gallery Plus mode, it needs to be created and uploaded at 1:1 aspect ratio (to fit the box exactly). That way, no stretching.
Since you are creating a composite image from two separate cropped pictures, I would create each individual cropped picture at 500px height x 250px width (2:1) and then combine the two to create a 500px by 500px (1:1) final image. Getting either the cropping at 2:1 or the pixel dimensions correct, is probably more a factor of your image rendering software's capability. Some products have that flexibility.
Danno