Change the Alt tag on inserted pictures in discussion

I have inserted some photos from my website inside the discussion and the Atl tag is the link to the photo. I want to change this to be the name of the photo file.

My second question is why when i insert photos inside the discussion they are not served over my CDN? Is this because all the inserted photo should be from external link and you didn't create this in mind someone will insert them from his website images folder?

Thanks