I think it would be a good idea, to give the administrator in the ACP the chance to choose where the uploaded images are stored.
When the webhoster has limited storage, you may want to host the images on one of the cloud image hoster
Also great would be to link each category with an external cloud hoster.
So you can decide, if category "cars" should store images on the local hdd or if the images should be stored somewhere else and only be linked.
The forum member shouldn`t take note where the images are stored.
For him uploading always feels the same