Use the circle shape tool to make a circle or ellipse and then move the shape so that only a quarter of it is visible in the corner. Crop it to remove any empty space that you don't need. Delete the background layer so that it's transparent and then save for web as a transparent gif file.
To make all four corners, you can just flip and rotate the image to get what you want.
In your HTML table, just create an extra row at the top and an extra row at the bottom. Create two cells for each of those two rows and put the four corner images in them. Remember to align them left or right so that they're flush with the edges of the table. You'll probably also want to set cellpadding and cellspacing to 0.
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