Search engines don't put much emphasis on keywords these days. Content is king. If you just have a jgp file then they will just see a jpg file and not much else, even if you have metatags, the won't help much.
You should have an alternative description in the image tag to tell people about the image should it not load in or should the people be a Search Engine spider or a blind person with a screen reader. e.g.
<img src="graphic.jpg" width="300px" height="300px" alt="A short description goes in here using keyword ritch text">