Upload forms are often precise about dimensions and reject anything that misses, even by a pixel.
If you force a width and height that do not match the original proportions, the picture stretches and anything recognisable in it looks off. That is why the shape lock is on by default.
When a form insists on exact dimensions that do not match your image, crop to the right proportions first and then resize. Two clean steps beat one distorted one.
Making an image larger cannot add detail that was never captured. Enlarging a small image will always look soft.
Yes. Every image you add gets the same target size.
Not with the shape lock on, which is the default.
No.