1. A computer system, comprising: an interface circuit configured to communicate, via a network, with an electronic device; memory configured to store a program module and a set of reference images; and a processor, coupled to the interface circuit and the memory, configured to execute the program module to perform operations comprising: accessing, in the memory, the set of reference images that include content with instances of a quantitative feature attribute and one or more feedback metrics associated with the instances of the quantitative feature attribute, wherein, for a given reference image in the set of reference images, the one or more feedback metrics specify at least a location of at least an instance of the quantitative feature attribute in the given reference image and a numerical value associated with at least the instance of the quantitative feature attribute, and wherein at least the instance of the quantitative feature attribute includes multiple instances and the numerical value includes a count of the multiple instances of the quantitative feature attribute in the given reference image; generating a machine-learning model based on the set of reference images and the one or more feedback metrics; receiving, from the electronic device, an image having additional content; analyzing the image using the machine-learning model to perform measurements of one or more additional instances of the quantitative feature attribute in the image; and providing a measurement result for the image, the measurement result including a second numerical value associated with the one or more additional instances of the quantitative feature attribute in the image.