There is, it seems, always (or almost always!) a conflict between good HTML practice and what someone, somewhere, has said is best for SEO. I tend to err on the side of good HTML because that is always right whereas, more often than not, many of the gimmicks designed to get you better search engines results don't seem to work as promised. Look at what happened to so many of the Google change known as Panda.
The purpose the HTML alt (alternate text) attributes in image tags is to provide, as the name suggests, alternate text that will be displayed when the image is not available. This means that if your image fails to show for some reason people looking at your page will at least know what was supposed to be there. The visually impaired also rely on the content of the alt attribute as web page reading software will use that to say what the picture is. That's what it's there for.
Some people suggest that you gain position in the SERPs by padding alt attributes with lots of keywords. I'm not sure that is true and, in fact, you can do your results a bit of harm by over-padding. Everything in moderation!
If your pictures are actually furthering the purpose of your website... that is, they are descriptive images that enhance the theme of the site/page or they are views of products you are offering for sale, then matching, alt text (a keyword rich relevant description of the image) will serve you best for both SEO and the actual purpose of the alt attribute. If neither of these is true then you should probably just remove the image!
Click Your Favorite Social Media Site Icon To Share This Page/Post With A Friend...