The C iii] 1909 (hereafter C iii]) line is the strongest ultraviolet emission line after Ly alpha and is therefore of interest to high-redshift studies of star-forming (SF) galaxies near the epoch of reionization. It is thought that C iii] emission is strongest in galaxies with subsolar metallicity and low mass; however, spectral observations of numerous such galaxies at high and low redshift produce inconclusive or even contradictory results. We present the first-ever C iii] imaging, obtained with HST/STIS for the low-redshift SF galaxy Haro 11. Cluster parameters like stellar mass, dust fraction and attenuation, and ionization parameter, obtained through spectral energy distribution fitting, show no correlation with the C iii] equivalent width (EW), which may be due to a combination of the limitation of the models and the age homogeneity of the cluster population. Comparing the ratio of C iii] emission line flux from individual clusters to that of H alpha, [O iii], and [O ii], we find that the clusters with the highest EW(C iii]) can be reconciled only with Cloudy models with an extremely high C/O ratio of >= 1.4(C/O) for an ionizing population of single stars, binary stars, or a mixture of binary stars and active galactic nuclei. Given the pointlike nature of strong C iii], the integrated total strength of EW(C iii]) becomes dependent on the morphology of the galaxy, which would explain the large scatter in EW(C iii]) strengths observed in galaxies with otherwise similar SF properties and of similarly low metallicity and stellar mass.