Have a look at [`ol.source.DebugTile`](https://github.com/openlayers/ol3/blob/master/src/ol/source/debugtilesource.js) for some useful ideas. In short, you'll need to override `getTile` in `ol.source.Zoomify` to return `ol.source.ZoomifyTile`s, and these `ol.source.ZoomifyTile`s will need to override `getImage` to return a larger image if necessary. Note that you can return the 256x256 canvas element directly, you don't need to do `image.src = canvas.toDataURL()`.