Alt tag or Title tag?

If you are not using "alt tag" then it's no meaning of your image, It's the biggest SEO mistake because google robot/crawler can't read images that have not text/alt tag and your images not rank in search engines.
Using title in 'alt tag" is a good strategy. But keep in mind if your post is lengthy and you use multiple images. Then You can use different keywords for the title and the alt tag.