You can use long tail keywords. For example your keywords are seo india and seo company then you can use seo services company india.
Simply describe the image or the purpose of the image - do not get het up about keywords. Remember this is the text people see if they have graphics switched off or the image fails to load. There for a button that says 'click here' as an image should have an alt that says 'click here to see XYZ'