My friend once told that I should include big image on the main page of my website. And it will be good for SEO.
So does image on the page improves SEO rankings?
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Yes and no.
It is content that ranks and image tags offer some to the total performance of the page, however, it is limited.
Where your friend is right is engagement. Google looks for elements that offer engagement. An image is the most basic engagement there is. At one point, Google was not shy about saying that an image should exist at the top of the content. SEO's agreed for several reasons and so for almost every page you will see on the web, there is an image at the top of the content.
So besides the value of the alt text and any potential that the path and file name give, Google credits the image as engagement. However, this too is limited.
From a user experience (UX) perspective, it does cause a pause in the users visit and may entice the user to read your content. It is just one element, however, it is the UX effect that is the greatest. It can extend the time spent on page and reduce bounce rates somewhat which are other metrics Google can use to rank a page.
Google robot is text crawler - so it can't "see" image (ony the code of image). But big image could slow down you page - and that can give you some negative ratings (you can measure it here https://developers.google.com/speed/pagespeed/insights/)
On the other hand there is User Experience ( you are creating website for people, not for robot) , and IMHO it's more important to have a good looking site(even with a ton of graphics) with a good content than plain text site with better seo rating :D .