A question about image file names and alt tags in blog posts.

For example lets say my 1,000 word article is about acne and my main goal is to rank for the long tail "how to treat acne overnight". So for SEO I use the title "How to Treat Acne Overnight Using Simple Methods", and then I use "how to treat acne overnight" somewhere in my first 2 sentences, and then once more in the article.

Now should my file name be called "how-to-treat-acne-overnight.jpg? Or should it be my title name? And what should I use for an alt tag? The title of my post or the long tail keyword?

Also I will only be using only one main image in my post since it's only 1,000 words at most. Thanks for any help.