Why does it say "v'El Mi Tedamyun Kel"?
Radak: You think that just like your gods have no power to save, also Kel. Heaven forbid to compare them to Him! The idol that you deify, it is man's handiwork.
Malbim: You doubt Hashem's ability to fulfill His promise. You speak about Him like a physical being of limited power - what He can do, and what He cannot; what He wants and what He does not want, and if His desire will change. You compare Him to existing matters, just you say that His physicality is higher than that of other matters. Based on this, you make images and pictures, and consider bowing to them like bowing to Him. Fools! There is no comparison to Him! He has no form or picture. Rather, He is elevated power and intellect without limit.
Why does it add "u'Mah Demus Ta'archu Lo"?
Malbim: Even based on your folly, that you consider Him to have a form, and there can be a semblance to Him - you should make an elevated semblance, like one of His higher servants - an angel or sphere. (Why do you make images of gold and silver? - verse 19)