The Alt tag is probably the most important image optimization factor. Alt stands for alternate. The text in the alt tag is displayed in place of the image if the image can not be shown or is taking too long to load.
Most of the major search engines are text-based, i.e they can not read images and videos. The location of your images appears as an empty space to the search engines. These empty spaces are not in any way good for your overall SEO. The alt tag not only tells the search engines that there is an image placed here but also informs it about the topic of your image.
A good practice for the ALT tag is to keep your alt tags short but descriptive. Fill your alt tags with search engine friendly information but don’t try to go too far.
Image size is a very crutial factor for your overall user experience. Images can make your page heavy. Put up a webpage that takes more than 10 seconds to load, and your users will escape faster than you can say “back button”. After the recent Panda update, the loading time of pages has become a vital part for your SEO ranking as the total time of visit has become more important as a SEO algorithm. So its important to keep your images low in bytes. Yahoo Smushit is a great tool to wipe out the excess bytes from your images making them lighter.
Just as keywords are important in url, the same goes for images. Keyword rich file names can help a SE determine relevancy. Avoid using default image file names like, “DSC0019.jpg” as it does not give much information about the image. Use descriptive filenames with keywords separated by “-” (hyphen). For example, “airtel online recharge”.