To avoid unacceptable image distortion, I'd suggest either beginning with an image that's close to a multiple of 40x50, or else cropping it to approximately those relative dimensions, before you use that online tool to create a 40x50 image.
(gem) Hvis du ikke ønsker at andre kan se hvad du laver, kan du skifte over til "cloak mode" i indstillingerne. (Kun for betalende medlemmer) (pauloaguia) (vis alle tips)