Many web pages contain elements such as menus, titles and even descriptive text, such as graphical images. These many varieties of elements can make your web page look very nice and attractive, but they are not very Search Engine Friendly. Search engines cannot Index the text that is contained within fixed images, and so this form of data usually makes no sense to their spiders.

Example:
The following {ShowGlossar:301}:

<IMG SRC="imagefile.gif">
Should be written as:

<IMG SRC="imagefile.gif" ALT="Image descriptions for engines to index">

In order to make your images friendly to most search engine spiders you should always ensure that any important text to be displayed in an image is contained within an Alt tag, which has either a copy of the text, or a Description of the image that is to be displayed

Tips.

Be sure to use the keyword phrases that you also used in the copy of your page, Title tag, meta description, and other tags.
Do not try to cram a bunch of keywords into the "alt" attribute. We recommend using no more than 2-3 per image.
Describe the image - do not just list keywords.
The "alt" attribute is also a good place for Misspellings and plural keyword phrases that you may not have used elsewhere.

By using

<IMG SRC="imagefile.gif" ALT="Image descriptions for engines to index">
your alt tags is showed when you mouse over the image.

To prevent this insert a empty title tag like this

<IMG SRC="imagefile.gif" TITLE=”” ALT="Image descriptions for engines to index">