In order for a link to be readable by screen reader users links must all contain either plain text, or in the case of a linked image, alt text describing where the link goes. If the linked image contains text, a good general rule is to make the alt text the same, or similar to the text in the image.
Screen reader support for reading the title attribute for descriptions of images is inconsistent and should not be relied upon as the sole means to convey important information. However, it is alright to use the title attribute in conjunction with an alt attribute.
A plain text link.
<a href="#">Get Help</a>
A linked image with appropriate alt text.
In this case, screen readers will announce the alt attribute to the user.
<a href="#"><img src="images/get-help.jpg" alt="Get Help"></a>