One of the most enduring misconceptions about search engine marketing is that whomever has the largest advertising budget wins. Although a larger advertising budget can certainly be advantageous, especially when targeting highly competitive keywords, but it’s far from a requirement for success with search engine marketing. This is because all ads go through a process known as the ad auction before appearing alongside search results. For the purposes of this explanation, we’ll be focusing on the ad auction in Google AdWords.
Another reason is that if you're using an image as a link, the alt text for that image will be treated similarly to the anchor text of a text link. However, we don't recommend using too many images for links in your site's navigation when text links could serve the same purpose. Lastly, optimizing your image filenames and alt text makes it easier for image search projects like Google Image Search to better understand your images.