(If you have the image as a file, just slap it on a cloud server of your choice and copy the URL)
For the second point, I assume you are referring to a HABPanel screensaver:
(This is pretty in depth and should show you how to do your slideshow)
Let me know if you have any questions regarding integrating the NAS to the screensaver, as that probably isn’t covered in the documentation but should be pretty easy to figure out.
P.S - I wouldn’t recommend placing an iPad in the wall, it’s a little overkill if you only want to use it to run a browser to display HABPannel, or even if you want to use other basic features. Get yourself a cheapish android tablet (not too cheap, it needs to be fairly reliable and not crash when you ask it to do one thing) I’m fairly confident that is a better option for your needs, and it will save you quite a bit of money.