X-ray microtomography (mu CT) scanning provides high-resolution images in applications ranging from medical to material sciences and failure analysis. In general, CT scanning relies on X-ray absorption to produce a 3D computed image of the material. In Earth Sciences, mu CT scans are used to characterize porosity and pore size, shape and topology of rock samples. For sufficiently large pore systems, the resulting segmented images may be used for quantitative transport calculations. In this note, we infer the limitations of mu CT images of rock samples, caused by attainable resolution for a representative sample size. To this end, (1) we perform a systematic analysis with the aid of a resolution chart, (2) we present example scans of an Indiana limestone and a Berea sandstone mini-cores, and (3) we process and analyze the images to extract pore structures using different segmentation algorithms. Porosity estimates inferred from mu CT images tend to be lower than bulk measurements.