For some background on serpent symbolism in Ancient Near Eastern (and particularly Israelite) religion, see: http://www.jehovahs-witness.com/10/73244/1.ashx
The prohibition on images is but one part of the comparatively late (7th century BC) authoritarian reform of Judean religion initiated by the Jerusalem monarchy (especially under Josiah). The Bible texts (written after this reform) give a rather confusing picture as they read the "new rules" back into the foundational times of "Moses" (notably in Deuteronomy), so they have to explain why a reform was needed in the first place. The usual "explanation," of course, is foreign influence and infidelity. But central features like the copper serpent on the pole right in Yhwh's temple have to be rationalised as an original institution of Yhwh (whence the story in Numbers) which was later "perverted" in an "idolatrous" way and then discarded. Others like the golden kerubim, which were not affected by Josiah's reform, are never criticised.