Image-related information can be provided for by using the "alt" attribute
Images may seem like a straightforward component of your site, but you can optimize your use of them. All images can have a distinct filename and "alt" attribute, both of which you should take advantage of. The "alt" attribute allows you to specify alternative text for the image if it cannot be displayed for some reason (1).

(1) Our image wasn't displayed to the user for some reason, but at least the alt text was.

Why use this attribute? If a user is viewing your site on a browser that doesn't support images, or is using alternative technologies, such as a screen reader,  the contents of the al t attri bute provi de information about the picture.

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.

Store files in specialized directories and manage them using common file formats
Instead of having image files spread out in numerous directories and subdirectories across your domain, consider consolidating your images into a single directory (e.g. brandonsbaseballcards.com/ images/). This simplifies the path to your images.

Use commonly supported filetypes - Most browsers support JPEG, GIF, PNG, and BMP image formats. It's also a good idea to have the extension of your filename match with the filetype.

Use brief, but descriptive filenames and alt text
Like many of the other parts of the page targeted for optimization, filenames and alt text (for ASCII languages) are best when they're short, but descriptive.
Avoid:

• using generic filenames like "image1.jpg", "pic.gif", "1.jpg" when possible—some sites with thousands    of images might consider automating the naming of images  
• writing extremely lengthy filenames  
• stuffing keywords into alt text or copying and pasting entire sentences

Supply alt text when using images as links
If you do decide to use an image as a link, filling out its alt text helps Google understand more about the page you're linking to. Imagine that you're writing anchor text for a text link.
Avoid:

• writing excessively long alt text that would be considered spammy  
• using only image links for your site's navigation

Supply an Image Sitemap file
An Image Sitemap file can provide Googlebot with more information about the images found on your site. Its structure is similar to the XML Sitemap file for your web pages.