Here's a quick way to make those arches:  I spent too much time doing Peg's instructions so I put my thinking cap on and came up with this way which took me forever to figure out because I did it but I forgot how I did it..

1). Start with your image at 300x400 as instructed in Peg's Tutorial.  (I used a black background because its easier to see what I'm doing).

2). Add a new layer.  Change your background layer to white and using your preset shapes draw a circle roughly about 102 x 102 in diameter.  Duplicate the circle 2x so you have three circles.  Align them side by side about 1/4 " apart enough so you can make your poles and merge them together.  Go to selections, select all and save them to Alpha Channel. Since there are only two channels in this quick tip, you can name them or just save them as is.  See the screen cap below.

3).  Add a new layer and using your selection tool set to rectangle, draw a rectangle above the circles.  You need to make the rectangle wide enough so that you can add your poles onto the sides. Make sure the edge is about the same width. 

4).  Open up your layer palette. Load from Alpha Channel the three circles.  Click on the top layer with your magic wand, go to selections/invert.  On your layer palette, click on Layer 2 and delete, click on your merged layer and hit delete again.  Once you have done this you can delete your merged circles.  Layer 2 becomes your arches. 

5). You will notice you have jagged edges. To fix this,  go to selections/select all and then selections/modify/feather/3 pixels, selections/invert and hit delete a couple of times.  This will make the edges a bit more smoother.  Now you can continue on with Peg's tutorial. 

As you can see from my original image I didn't use this procedure to get rid of the jagged edges because I already saved mine as JPG. 

Good Luck and Have Fun with this one!!