The alt and title attributes should align. You cannot set the alt text to null and use the title attribute. If the image is meaningful- the alt text should complete the title text. In case the image is decorative and should be ignored by assistive technology the title attribute should be removed or left empty.
Engine scans the page for
<IMG> elements (excl. images inside links) with empty alt and non-empty title
Users who use assistive tools, such as screen readers rely on a descriptive information to detect the content. Incorrect use of the attributes causes confusion for those users.
<img src="/example_"img.gif" width="20" alt="example_" />