The [IMG'] attribute that shows a image from another site usually get 404 erros and this is bad for every website.
I also think that an ALT field would be awesome when someone posts an image. Ofc this field would be named as "image title" and then the user could place the ALT attribute thinking it as a title.
So my suggestion is, once someone clicks the image button on redactor to place a link:
- ALT attribute field (obligated or not to be choosen on admin CP).
- Image name
- Host the image on forum's base (as attachment or not, I don't think it really matter). Once marked, the link posted by the member will be a refference of where the image might be downloaded.
- Image size limit
Well, for now these options are enough but they could be enhanced by given the user an option to place the image with a specific size in % in proportion. So if the image is 1000x1000 and the user inform 10% it would post the image as 100x100.
Those options based on permissions, so I could apply only for specific user groups.