You can do that with JPEGs, just not the obvious way (i.e. by exporting the JPEG at the target resolution.)

Instead, you need to export the JPEG at a resampled resolution that's a multiple of your target resolution, such that each pre-transform source-image pixel gets mapped to its own entirely-independent JPEG color block.

Most obvious (though perhaps not optimal?) approach: nearest-neighbour upscale your image by 8x, and then save as JPEG with 100% quality (which will create 8x8 blocks with 4:4:4 subsampling.)