Forum Moderators: open
These tags aren't really heavily used by the SEs, but having them can't hurt. It may help a little by increasing the occurrence of your keyphrases in the page's coding.
Even when an image is linked, I use one or the other, but not both.
Make sure that your TITLE and ALT tags are descriptive and accurate.
And then we have the title attribute which can be used on various elements and acts like a tooltip. Based on the original question, I'd guess we are talking about the title attribute and not the <title> element.
I do believe the above reference from Google is referring to the <title> element and not the title attribute. Based on many topics concerning the title attribute, it does not appear to be something that can be used to influence page relevance. It is more of a usability issue than anything else. It also adds a little spice to those links that may not be properly worded. You know, those Click Here links that give no indication of where the link will take you.
Make sure that your TITLE and ALT tags are descriptive and accurate.
This is an interesting quote. It sounds like G is referring to the attributes since there is not, as far as I know, an ALT tag.
However, taken at face value, it does seem to be talking about the <title> tag, not attribute.
Ultimately, I suppose it doesn't matter, since all three really ought to follow the above advice.
whether to use title tags *and* alt tags on non linked images please
Using alt attributes on your images (linked and not linked) is a good idea for accessibility reasons, if nothing else. Without them, visually impaired users will not know that the image exists at all (a particularly bad idea if the image happens to be a link!). If you have to decide between the two, I would go with alt.
cEM
7.4.2 The TITLE element [w3.org]
There is no "official title tag".
7.4.3 The title attribute [w3.org]
Values of the title attribute may be rendered by user agents in a variety of ways. For instance, visual browsers frequently display the title as a "tool tip" (a short message that appears when the pointing device pauses over an object). Audio user agents may speak the title information in a similar context. For example, setting the attribute on a link allows user agents (visual and non-visual) to tell users about the nature of the linked resource.
Based on the original question, if the image is linked, you would utilize the title attribute on the <a href> and not the <img> tag. For example...
<a href="file.asp" title="Short Descriptive of Linked Resource"><img src="file.png" alt="Less than 80 characters to describe the image" />
Alt Attribute [w3.org]
Also referred to as the Alt Tag or Alternative Text.