1. A computer-implemented method for an image-based analysis of a geological thin section, comprising: (i) acquiring a plurality of images from a geological thin section of a rock sample from a subterranean zone, wherein acquiring a plurality of images from a geological thin section of a rock sample from a subterranean zone comprises: acquiring at least one plane-polarized image from the geological thin section by acquiring four plane-polarized images from the geological thin section, each plane-polarized image rotated at a distinct angle relative to a zero position angle, from the geological thin section, and acquiring at least two cross-polarized images from the geological thin section by acquiring four cross-polarized images from the geological thin section, each cross-polarized image rotated at the distinct angle relative to the zero position angle, from the geological thin section; (ii) manipulating the plurality of images to derive a composite image, wherein manipulating the plurality of images to derive the composite image comprises: registering the plurality of images, the registering comprising determining a rotational center for each of the plurality of images about which to apply a rotational transformation, applying the rotational transformation to each of the plurality of images with a rotation angle equal to an opposite value of the distinct angles relative to the zero position angle, generating a first composite image, generating a second composite image, and generating a third composite image, wherein the first composite image is generated by applying an edge detection algorithm to the at least two cross-polarized images, and the first composite image comprises an aggregation of a plurality of first composite images, with each first composite image corresponding to a particular one of the plurality of cross-polarized images; (iii) optimizing the composite image to derive a seed image; (iv) identifying, in the seed image, a particular seed pixel of a plurality of contiguous pixels that comprise an image of a grain of a plurality of grains of the rock sample in the seed image; (v) determining, with a specified algorithm, a shape of the grain based on the seed pixel; (vi) determining, based on the shape of the grain, a size of the grain; and (vii) preparing the determination of the size of the grain for presentation to a user.