G'day, Dave, and welcome to
Kunena.
The short answer to your question is no; there is no resize-on-the-fly-during-file-upload facility for
Kunena.
However, thank you for a very interesting question and asking for our opinions.
I am sure that we would all like some way to have people add as much as they would like to add to our websites, but to still retain control over how much disk space their uploads consume. This is a tough question to answer.
The answer largely depends on three things:
(1) How important it is to the business of the website to have high-quality images uploaded by the users;
(2) How much disk space is offered by the web-hosting provider when a person uses those services in building a website; and
(3) How much money people may be prepared to spend to have large or (virtually) unlimited disk space to accommodate all the images that they would like their users to upload.
These are impossible-to-answer questions because the circumstances vary from case to case.
While it is not possible to have
Kunena resize images "on-the-fly" when they're uploaded, you can control several things:
(1) Whether or not you will allow registered users and/or guests to upload images;
(2) The physical dimensions of those images,
viz. height, width and file size; and
(3) The ability for the site administrator or moderator to remove images from the forum if they do not fall within the site's terms of use policy or for any other reason.
It would be wonderful, wouldn't it, if people did not have to actually have manage a discussion forum on a regular basis. Unfortunately there are few facilities that I've seen these days that eliminate the need for some kind of "housework".
Nothing is impossible but everything has a price, as the saying goes. If this is not the answer you were looking for, then I wish you every success in finding a product that will achieve what you are looking for that results in a web-based discussion forum that is relatively hands-free. I haven't found one like that, yet.
There are, of course, other options. People can upload their images to any one of hundreds of free hosting services on the internet and link those images in their messages on the forum. This is a no-cost solution for your clients because all the costs are borne by your client's users who have to find somewhere to upload their images and then perform the necessary linkage when they post their messages on your client's forum. At least your client won't have to worry about any "housecleaning".