We propose to obtain [NII]Lambda658.3 nm images of the extended nebulae around the symbiotic binaries CH Cyg {HD182917} and He 2-104 {PK315+09D1} using the WFPC2 camera {+ [OII]Lambda372.8 nm with STIS for CH Cyg}. These large ionized nebulae, one of which has been only recently discovered, trace mass loss episodes spanning a wide range of time scales. In particular, the marked bipolar morphology of the nebulae makes them a very attractive laboratory where to study the effects of binary interactions in producing highly collimated nebulae and jets, and where to test current dynamical models of common envelope evolution. This has important implications for several astrophysical topics {e.g. shape of planetary nebulae, ejecta of classical novae, jets from young stars, efficiency of common envelope mass loss, frequency of formation of white dwarf pairs as the precursors of SN Ia}. Even in the best ground-based images, these symbiotic nebulae are often poorly resolved, especially in their inner regions. A precise morphological characterization from HST images, coupled with spectroscopic data from the ground, is therefore the first and fundamental observational information needed to understand their nature and that of their progenitors. Moreover, with HST we have the unique opportunity to follow in real-time the spatial evolution of the outflows {on a time basis of very few years}, and therefore to derive 3-D dynamical models of their structure.