The resonance structures and formal charges are shown below:
Notice that the chlorine dioxide molecule has an odd number of electrons (19). None of these resonance structures satisfies the octet rule; in each of these resonance structures, one of the atoms has an odd number of electrons around it. Formal charge arguments predict that the two resonance structures with the odd electron on O are most important (these give the lowest formal charges.) This is a little counterintuitive - we would expect the more electronegative element (oxygen) to have the full octet.