Can Mirror Image be discarded using Decree of Strength?

Yes — Mirror Image can be chosen by Decree of Strength, but it isn't discarded; it's recycled.

Decree of Strength lets you pick a Mind ([C]) card from your opponent's revealed hand. Mirror Image is a Mind–Order spell, so it qualifies. However, the card doesn't go to the trash — Recycle puts it on the bottom of its owner's Main Deck (Rule 416.1.a), whereas Discard is strictly hand → trash (Rule 422.1). So the effect works, just with a different destination than "discard."