Please, "alt attribute" or "alt text". "Alt tag" is wrong and can be confusing as soon as people want to move on to better understand the W3C specifications and tutorials.

The original image has a keyword rich name
Only really useful when written as English. Foreign languages and digital camera file names are much less useful than the alternative text used to name the image. The "title" attribute can describe the image and aid further to the indexing robots.

Always use "alt' text for important images.