Welcome to the Q&A Forum

Best practices for on-page SEO say that the alt text for an IMG tag should be equal to the keyphrase that the page is optimized for. So if you are optimizing a page for 'blue widgets' then having an IMG with alt text = 'blue widgets' would be good. You may also want to name the image file blue-widgets.jpg (or blue-widgets.png or .gif). It's one of several on-page signals you can send Google that this page is all about blue widgets.