If I change the images on my homepage very often, will Google like it as they like content update(text)? Wil I rank better than a competitor site that never changes its images?
I dont think if changing image will just make change to your content and that google will consider it as content update.
yes Google does not give importance to image. It always gives importance to alt tag but if you change your web site home page image & you put wrong alt tag on your image than it surely effect on your web site & ranking !
Images has an less effect in SEO.But i think everybody should concern about image "alt". because mass people also search by using image option.It will help you getting some traffic.
How does google know it is the wrong alt tag? As it stands I do not think Google can decipher an image and does simply rely on the alt tag and possibly image name/url
If you change image on any site its not necessary you have to change content, atleast not for search engine.