Actually, if you want to put alt text onto your images you do the following : right click image, a menu will open, choose Web Properties, choose Images and then write your SE friendly description in the Image Caption (Alt Text) section. You can also just click on the LINK image (looks like a chain) and it will also open the web properties box, then just choose Images etc. When you publish the site, if you right click on the image it will come up with the alt text, and this is the text that the google spiders will find when crawling your site.