There are no text hints for ALT attribute of IMG tag; and for TITLE attribute for over tags.
Not having tooltips for alts was a design decision (alts are replacements for text, and title can be used if tooltips are desired). Title not working may be a valid bug, though.
This design decision is made clear between bug 1994, "IMG alt text should go inline when image cannot be displayed" (for ALT) and bug 1995, "[RFE] metadata attributes not accessible to use" (for TITLE) (see the 1999-05-21 10:41 comment). I hesitate to DUP against a multi-issue bug, especially since some parts of bug 1995 are 4xp and others are RFE. TITLE tooltips are 4xp. Ian?
Remembering to cc Ian this time...
*** This bug has been marked as a duplicate of 27828 ***
Beats me how I managed to miss spotting that one ... obviously my query was too narrow. In any case, this is indeed a DUP of bug 27828, "TITLE attribute should show tooltip". VERIFYing.