As @usher.br mentioned, Apple controls what is possible on an iOS device via their MDM framework. If the capability to manage the device wallpaper isn't there, JAMF can't provide the ability in their product. There's no real leeway with this, and I've heard of cases of some MDM providers losing their right to manage iOS devices for trying to get around Apple's restrictions and policies.
Without even looking into it at all, I can tell you that something like the device wallpaper is seen by Apple as a very personal setting, so I would not hold my breath on seeing the ability to control that coming anytime soon. Apple is wary of stepping on what they see as personal settings on iOS devices.
I hadn't heard that it was mentioned in the 9.64 release notes. If its mentioned there as a capability, you should contact JAMF support to find out what that references. I'm wondering if that wasn't supposed to be in there, or if it only shows up under very specific circumstances. Not managing iOS here, so I have no point of reference myself.
Ok @mm2270 just jogged my mind with the last post. So I looked. You can only do this AFTER the devise is Supervised from Management tab, then Management Commands. There is a new button "Set Wallpaper". You can set both, or just one or other. "Image or photo to set as wallpaper. It is recommended that you use a file with the JPEG or PNG format"