I would suggest against using the image as a background for your work. To get the edges you speak of is (like most cool HTML tricks) rather simple and has a nice effect. Because I use tables my explanation will be in tables and may need some adaptating to other forms.
Step 1
Create 1/4 of a white circle image in your image editor (black background). You can do this by creating the full circle (hold down shift while drawing the selection in photoshop so the shape is consistly round) and then cropping to the one fourth corner. Save this image as corner1.gif. Repeat the process by created all the corners and naming them respectively. If you know how to rotate or flip and image - do that!
Step 2
Make an HTML document with a black background. Create a table that has 3 rows and 3 columns (cellpadding and cellspacing set to 0). In r1, c1 place your first corner. In r1, c3 place your second corner. In r3, c1 place your third corner. In r3,c3 place your final corner. Typically you would place another table inside r2,c2 with the cellspacing and padding that you choose (if any). In this new table you can put all the information you want. Make sure you go through and set the background color of the TD tags to white.
Side Note: Some people make the space around the corner in the GIF transparent for use on different backgrounds. This can be useful.
There are many other ways to acheive this and by no means do I declare my example the right one.
Hope this helps.
-OMP
