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IZw18, ever since regarded as the prototypical blue compact dwarf (BCD)galaxy, is, quite ironically, the most atypical BCD known. This is because itslarge exponential low-surface brightness envelope is not due to an old stellarhost but entirely due to extended nebular emission (ne) (Papaderos et al. 2002;P02). We study IZw18 and IZw18C down to an unprecedently faint surfacebrightness level using HST ACS data.  We argue that the properties of IZw18C can be consistently accounted for bypropagating star formation over the past ~100 Myr, in combination with stellardiffusion and the associated radial stellar mass filtering effect (P02).  As for IZw18, we find that ne extends out to ~16 stellar scale lengths andprovides at least 1/3 of the total optical emission.  The case of IZw18 suggests caution in studies of distant galaxies in dominantstages of their evolution, rapidly assembling their stellar mass at highspecific star formation rates (SSFRs). It calls attention to the fact that neis not necessarily cospatial with the underlying ionizing and non-ionizingstellar background, neither has to scale with its surface density. Theprodigious energetic output during dominant phases of galaxy evolution mayresult in large exponential ne envelopes, extending much beyond the stillcompact stellar component, just like in IZw18. Therefore, the morphologicalparadigm of IZw18, while probably unique in the nearby Universe, may beubiquitous among high-SSFR galaxies at high redshift. Using IZw18 as reference,we show that extended ne may introduce substantial observational biases andsignificantly affect fundamental galaxy relations. Among others, we show thatthe surface brightness profiles of distant morphological analogs to IZw18 maybe barely distinguishable from Sersic profiles with an exponent 2<n<5, thusmimicking the profiles of massive galaxy spheroids. (abridged)

I Zw 18 as morphological paradigm for rapidly assembling high-z galaxies