In earlier versions of Excel you could insert a comment on the worksheet, and then use the Drawing toolbar commands to change the comment's rectangle to a different shape. It's not a technique that you'd want to use too often, but it can add impact to a worksheet if used sparingly.

In Excel 2007, the Drawing Tools Format tab doesn't appear on the Ribbon when you select a comment, so the Change Shape command isn't available. It's still possible to change the comment's shape, but it takes a bit more effort.
Add the Change Shape Command
The first step is to add the Change Shape command to the Quick Access Toolbar (QAT):
- Right-click the QAT, and click Customize Quick Access Toolbar
- In the Choose commands from drop down list, click Drawing Tools | Format Tab
- In the list of commands, click Change Shape, and click Add

- Click OK to close the Excel Options window.
Change the Comment Shape
- Select the cell that contains the comment
- On the Ribbon, click the Review tab, and click Edit Comment
- Click the border of the comment, to select the shape.
- On the QAT. click Change Shape, and click on the shape that you want to use.

Watch the Video
To see the steps for adding the Change Shape command, and changing the comment shape, please watch this short Excel video tutorial.
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Thanks for the Tip, I managed to add this to the custom header.
As I am working very much with excel I find this very useful, Thanks for the great tips!
thanks, that was awesome!
Nice tip, thanks.
That's a great tip! I already knew how to add an image to a comment, but I hadn't seen this before. Nice post.