The propagation of a shock wave into a medium is expected to heat the material beyond the shock, producing noticeable effects in intensity line ratios such as [O iii]/Hα. To investigate the occurrence of shocks in planetary nebulae (PNe), we have used all narrow-band [O iii] and Hα images of PNe available in the HST archive to build their [O iii]/Hα ratio maps and to search for regions where this ratio is enhanced. Regions with enhanced [O iii]/Hα emission ratio can be ascribed to two different types of morphological structures: bow-shock structures produced by fast collimated outflows and thin skins enveloping expanding nebular shells. Both collimated outflows and expanding shells are therefore confirmed to generate shocks in PNe. We also find regions with depressed values of the [O iii]/Hα ratio which are found mostly around density bounded PNe, where the local contribution of [N ii] emission into the F656N Hα filter cannot be neglected. Tables 1 and 2 are available in electronic form at http://www.aanda.org