I'm trying to create a 'make your own' leaflet template for my sales team

As Joel notes, this can be done with forms magic, but the tech really isn't there to let users do much on their own. 

I'd do this a little more old school. Do a lovely layout and provide it in PDF. Then do a tightly locked Word doc as an editable overlay. Two passes through the printer, and voila. 

You could even commercially print the base page for quantity and quality advantages. 

We got a lot of mileage out of such preprint methods; despite things like forms, I think there's still room for the method.

The interactive PDF forms I usually work with are too complicated to be made with InDesign's inbuilt buttons-and-forms functionality. InDesign's tools in this area have been improving a lot recently, but I don't see any way to embed the Javascript you'd need to pull this off from within InDesign. 

However, if you're willing to do the work in Acrobat, and you have any background making forms in Acrobat, it's not that hard, I think. You'd go into Forms in Acrobat, make a new Button, set it to Icon Only in the Button Properties, and in the Actions you'd set it to Run a Javascript on Mouse Up, and the entirety of that JS is:

event.target.buttonImportIcon();

That spawns a little dialog where you can browse your drive and pick an image. There are lots of limitations; for instance, the button won't change shape, so your non-designers wouldn't be able to scale images or change aspect ratios or any of that. So maybe if you made them a button that was precisely sized to match the sizes of the images of all your products... in the unlikely case that every single product image is of the exact same aspect ratio. 

So, maybe this solution won't work for your use case. I have seen many responses to similar challenges over the years, but none of them seem to me to be the kind of thing that one InDesign-using designer implements alone. I've seen:

• InDesign Server installations mated to web interfaces that allowed for template tweaks & PDF production in-browser
• Carefully groomed InDesign Templates (.indt) where most of the page objects and layers were locked down, allowing the non-designers to boot up InDesign themselves and change things with relatively little risk of damage
• Adobe Livecycle PDFs (now essentially a dead tech, but you could have done this button-image-import tech ten years ago)
• You can place images into Word files and run 'em in and out with Em Software's WordsFlow; I think you can do the same with Excel

I'm sure more will come to me if I think of it; you're not making an unusual request, but there isn't one single answer that is easy to implement that will cover all of the many varied possibilities.