The image has to be less than 10 megabytes. We do recommend, though, that you send an optimized version of your image (typically less than 500 kilobytes) to reduce latency and optimize bandwidth usage.
If you ever need to reduce the size of images while preserving their quality and size, we recommend that you have a look at JPEGoptim for JPG images or OptiPNG for PNG images.
We're always looking for advice to help improve our documentation!