An Image control on a UserForm has a ControlTipText property, but that ptoperty isn't available for a control on a worksheet.
You could have your picture as the fill of a shape and add a hyperlink to the shape setting its ScreenTip to the message you want. If you hyperlink to the workbook on disk nothing will hapeen when you click the shape.
Thanks Andrew. I think I got the right thing, but it's not quite right!
The tip did move to under the image, but then trying it on a second image
it lost it's positioning again. But a bigger problem - if I give the workbook to other users I'm not sure how to set their hyperlink address.
I'll hunt around (unless you know) for a method to detect when the cursor changes to the hand. I could then (I hope) get the coordinates and what picture it's over and display the tip via a macro.