The coupling properties of multi-core fibers are analyzed using the multipole method and coupled mode theory in order to gain insights into the performance of image fibers that are commonly used in flexible endoscopes. It is explained that coherent fiber bundles with high core density are able to transport images because nonuniformity in the pixel size reduces the inter-core coupling that causes crosstalk. The wavelength, average core size and separation, and degree of core size variation determine the strength of coupling between adjacent cores, such that fibers with a smaller core size and separation at longer wavelengths require more nonuniformity in order for reliable image transmission. Guidelines are given for assessing the performance of image fibers in a particular system.