As far as I know there is not a way to do this. What I setup in my doc types or transformations is to use the image as a URL vs a direct upload or upload file. This way the user goes to select an images URL and I can then dynamically create the < img > tag. It would look something like this
<img src="<%# Eval("ImageUrl) %>" title="<%# Eval("FileName") %>" alt="<%# Eval("FileName")" />This forum post
talks about media library alt tags, maybe it will help.
Best of luck