Flickering/flashing when updating image or moving mouse ACC2007 Wi

I have an image control supported by a lot of code to allow dynamic and
interactive graphics - for example drag and drop of activity boxes in a Gantt
chart. I trap mouse up, mouse down and mouse move events, and I repaint the
image by replacing its PictureData. I developed it under XP and ACC2003 with
a lot of GDI and all was OK.

Now I'm on Win7, ACC2007. With the standard Windows 7 theme the image
flashes briefly to the form background colour (I think) when it is updated.
It doesn't flash every time - the frequency increases with a larger image but
there is no consistency. (But if you are dragging and dropping, that means
many flashes a second and a pretty rapid migraine - basically unusable.) I
happen to have another form open with a TreeView control in it - and the
TreeView flashes too, with the text and graphics briefly disappearing.

I just found out that when I change to Windows classic theme the flashing
stops in the form with the image. The TreeView carries on flashing.

And - possibly related, possibly not - if I detect via the mouse move event
that the mouse is over an activity and change the image ControlTipText
accordingly, then the image flickers again ... even though the PictureData is
not changed. That's with any theme. The TreeView doesn't flash this time.

Weird stuff. Any insights or tips gratefully received.