Adobe® Photoshop® CS3 Extended software and a photographic time series were used to generate accurate and precise measures of change in the area, perimeter and diameter of Rhizocarpon thalli at one, three and seven year intervals. Systematic measurements at a fixed grid of eight diameters per thallus showed a rapid and highly variable diametric growth phase in the smallest thalli (<5 mm2) and slower diametric growth (<0·01 mm2 yr–1) in larger thalli (5–500 mm2). When standardized to an annual rate, the areal growth trend was similar, regardless of the number of years studied. This suggests that the areal and diametric growth of small and mid-sized Rhizocarpon thalli may be insensitive to annual climatic variation and can be accurately characterized by repeat measurements taken over months rather than decades. Unlike diametric growth rate, change in thallus area and perimeter are statistically robust measures of growth in Rhizocarpon thalli. Our mean measurement accuracy was 99%. Measurement precision (reproducibility) was >95% (P>0·05) for thallus area and >96% for thallus perimeter. Our technique is tedious, but on flat rocks it can resolve and accurately measure change in thallus morphology at the sub-millimeter scale, and it can be used with recent and/or historical images.