Hi,
I really don't understand what you want.
You're talking about product picture and product description.
In HikaMarket you can upload product pictures. The pictures are vendor independent.
A vendor could not access to pictures of another vendor. The main vendor (the admin) could access to all vendor pictures.
The product description use an editor. The editor could be "tinymce", a raw editor or one editor from the long list of Joomla editors.
If the editor propose an upload system, the upload system is on the plugin/component itself. We can't make magic like that.
Kunena has his own upload system, like HikaMarket have for the product.
The vendor could use the product images that he has upload in his product description if the editor allows drag & drop.
Because you can upload images, you can list them too. So you can retrieve the image path and use it in the product description.
What I want to explain you is that, the upload in HikaMarket works good and jDownloads is not HikaMarket.
The editor for the product description should propose editing features, but for download features, I don't think that it is the right place.
Regards,