I can't answer directly to your question regarding the iso image, but using the 2.5 image and updating it isn't a big deal: after installing, you go to the admin panel, and update your system.
You would probably have to do it anyway because of the regular update (like security fixes) of the distribution. And even Yunohost get some minor updates from time to time.
So even if it's a bit less convenient, it's not a big issue.
(but in the future we will need a new image for Debian 9 / Stretch, as updating from the old one is another story, and less user-friendly)