Tell people in plain text elsewhere that clicking the thumbnail goes to a larger version.
I would argue the alt text there should say the purpose of the link, not the "text" of the picture, unless it's a gallery? Because when you have
the image is taking the place of what WOULD have been anchor text. (if it's a gallery then I guess it would need to say "larger version of this picture of puppies" and alt text on the large version just whatever's appropriate to that image)
In this case, a more sensible title attribute might be "Open large image" - that would tell the visitor what will happen if he clicks on the small image.
Quite evil to kinda give the finger to those who aren't using a mouse. With the explosion of mobile browsing this is just getting worse. Everyone should be told clicking on the image brings you to the large one.
Of course, browser vendors could make title accessible to non-mousers, but for whatever reason, they choose not to. So WE the developers are stuck fixing this. Which sucks balls.
If you don't want any tooltip at all, use title="". Don't just leave the attribute out, because IE, contrary to standards, will then use the alt attribute for the tooltip, which is certainly NOT what you want.
Sometimes I do this, but more often I've discovered regular IE users are expecting that tooltip... some people even use it to orient themselves. So many times I just leave it. People like their bugs.