The script uses the file name of the image file to create the content of the alt attribute. So just name your image files like the titles you wish to use.
Example: Filename -> My_holidays_in_Canada.jpg ALT -> "My holidays in Canada"
Update:
* The valid suffixes are now configured in the include.php. Allowed suffixes: *.jpg, *.png, *.gif * I set the z-index for the preview image to 1000 (needed this for a page using Superfish menu)