Rule: Use alt and title attributes on <img> tags
Use always alt or title attributes on image tags <img>. They are important for a good SEO result because they describe the content of the image.
The title attribute is more descriptive than the alt attribute but only the alt attribute will be shown to users if a loading problem of the image occurs.
For an optimal result use a short image descriptive alt attribute and a more detailed and relevant content with good keywords within the title attribute.
Rule: No HTML down scaling of images
It should be avoided to scale down (or up) images by the browser itself by set up a custom height or width attribute on <img> tags or by CSS definitions because they will be slow down the rendering process of the browser depending on the original image byte size.
An other benefit is a reduced byte size on native downscaling of the image. This will result in a faster page load time and less traffic.
Rule: No empty src attributes
Do not use empty src attributes because they result in another request of the browser to the server. The additional request will load the current document within the image container and will slow down the rendering process.
Rule: No spacer images
Do not use spacer images! Spacer images indicate bad html structure and less knowledge in modern web architecture.
Since 2001 a website can be layouted only with CSS. Before 2001 web layouts are often table based with usage of table tags.
Spacer images results in additional and unneccessary HTTP requests for the web server. It will slow down the render process of the page.
Rule: Avoid images for layout purposes
Use css image sprites instead of html <img> tags for layout purposes because they decrease the total HTML requests which results in a faster load time.
They indicate also a bad coding style and a minor knowledge of modern web architecture.
Rule: Avoid pure image navigations
Avoid usage of image navigation build with <img> tags because the SEO effect with plain text is much higher than an image navigation. Image navigations should be build with css background images and hidden navigation text within <a> tags if possible.
If the page must use an image navigation, the images should contain alt or title attributes!