Both of them
However, the alt description in the image should be for the link description, not for the image itself
From Accessibility and Usability at Penn State: Image ALT Tag Tips for HTML
term "ALT tag" is a common shorthand term used to refer to the ALT
attribute within in the IMG tag.
- Any time you use an image, be sure to include an ALT tag or ALT text
within the IMG tag. Doing so will provide a clear text alternative of
the image for screen reader users. WCAG 2.0 Guideline 1.1.1.—"All
non-text content that is presented to the user has a text alternative
that serves the equivalent purpose."
- The description in the ALT tag
should be meaningful in the context of the Web page, specifically:
- Images used as links should have alternative (or "alt") text
describing the destination of the link, not the image itself.
with acronyms should be written with spaces in between letters. For
instance, <alt="I T S at P S U" > (read by a screen reader as "ITS at
PSU") is preferable to <alt="ITS at PSU" > (read as "It’s at Sue").
- Images used as spacers or in toolbars should have an empty ALT tag
(i.e. <alt="" >). Screen readers will simply skip over images with
empty ALT tags.
- Images that already include a text description within
the main text of the page can have a summary ALT tag.
provide a tooltip for visual browsers, use the TITLE tag in addition
to the ALT tag, since it is supported in most browsers. For example:
- While there is no official length restriction on the length of alt
text, many experts recommend 125 characters or fewer because of
restrictions within the JAWS screen reader. Many versions of JAWS
break up longer text tracts into blocks of 125 characters, which can
be confusing to users.
- For an especially complex image, such as a
chart, equation or diagram, a link to an extended text description
should also be included.
- Images that are used as buttons or labels
should use fonts that are readable to a large segment of the audience
(probably 12 pixels/point or larger).
- In some cases you can replace
decorative or layout-related images with styled HTML elements, such as
HRs or DIVs, for which you change background colors and specify
background images.
The article above clearly explains the alt usage on image links. However, the title part is incomplete in the original article, but you should use both, because one of them will help you with assistive technologies and accessibility , while the title on links will help you with this, but also with better readability and SEO.
Additional concerns
On top of the above, you should never rely on title only, see The title attribute
Relying on the title attribute is currently discouraged as many user
agents do not expose the attribute in an accessible manner as required
by this specification (e.g. requiring a pointing device such as a
mouse to cause a tooltip to appear, which excludes keyboard-only users
and touch-only users, such as anyone with a modern phone or tablet).
but... by no adding it, you get this:
If this attribute is omitted from an element, then it implies that the
title attribute of the nearest ancestor HTML element with a title
attribute set is also relevant to this element. Setting the attribute
overrides this, explicitly stating that the advisory information of
any ancestors is not relevant to this element. Setting the attribute
to the empty string indicates that the element has no advisory
information.
In short
You must use both elements whenever possible, but there are some specifications and special rules you should follow
