When describing your image, keep it very brief. Use your keyword research when possible and applicable. Filling the alt tag with a description that does not match your image or your content is neither helpful for the user nor the search engine. Adding too many keywords to your image can also be damaging to your site if it is widespread as it can appear to a search engine that you are "keyword stuffing."
Keep in mind that this image can appear in a search engine's image database, like Google Images or Bing Images. If you are mis-tagging your image, you will either receive no visits or visits from a user who immediately leaves your website because the image didn't match his/her search. Below, you can see an example of what can happen if you don't flag your image accurately. A search of "Tucson Hiking" has displayed an image of Van Gogh's The Starry Night mixed in with mountains and hiking trails.
The title field can also be filled in with the same text or a unique description. The title field displays the text to the user, unlike the image alt tag which is only visible when the image is not displayed or to text-only browsers. The image's alt tag is more important than the image's title tag, so if you only want to fill out one field, complete the image description with useful, relevant information.
If your website is image heavy, adding alt tags to them all can be time consuming. Avoid feeling overwhelmed by grouping your pages in order of importance and handling them in smaller batches. Remember, patience and research are key to optimizing your website well for users and search engines!