Like other services (Netflix, YouTube, etc.), Zoom will adjust your bitrate to accommodate your upload speed.  If you have a slow upload speed, Zoom typically compresses more so that things still stay real-time but the video feed will look choppier the slower the upload speed.  They tend to just drop quality rather than lag behind.  Still images are usually easy to deal with because nothing is moving though so you may get away with it even on a slow upload.  The best way to do this is to do a test Zoom call.  You can do all sorts of calculations but you'll never really know until you test it and get the person on the other end to tell you how it looks.

Regards,
Mike