Resizing a photo to 1200x630 without stretching: why the result can be 840x630

A photo that looks stretched usually has a width-to-height ratio that differs from its original. Resizing a 4:3 photo into a 16:9 rectangle by changing both dimensions independently makes the objects in it wider or narrower. Keeping the aspect ratio avoids that distortion. It doesn't mean the result will fill every box you give it, though. That distinction matters when a website asks for an image at a specific size.