Althought using the current tooltip option will not allow you to add images and other objects, I have come up with another work around using another tootip tool (
http://www.walterzorn.com/tooltip/tooltip_e.htm) in conjunction with this tool.
Download the neccesary js files from the link above and follow the directions to get it working with your page.
In your code where you define the elements of the grid, do something like:
obj.onCellMouseDown = function(event, column, row){ Tip('someText') };
NOTE: I created the function here because I wanted some information about the cell where the mouse was being clicked.
where Tip() would be the javascript function to call to run the tooltip. I have run into problems using the mouse events (onCellMouseOver in particular) for running the script (the tooltip follows the mouse around the screen) but, if you use the "OnCellMouseDown" function where the user has to click on the cell in this case, it works perfectly. You should also pay attention to the elements that can be changed with the tooltip tool on that site. Will take your grid to another level.
Heres to good coding!