Would you like your icons to look higher than the panel? Do this:
1. Choose a background image from gnome-look.org.
2. Open it with Gimp, and set its height to 20 pixels. (Image>Scale Image)
3. Create a new file (File>New) that's 37 pixels in height, keeping the same width of the image you've just resized.
4. Go to Colors>Color to alpha and click on Accept.
5. Now you have a transparent background. Copy and paste the resized panel image on the transparent background. Select the Move tool in the left Gimp panel and move the pasted image all the way down. Now you have a 37 pixel high background image, where the upper half is transparent.
6. Save the new image in the png format (you have to specify this format in Gimp, or it will save it in the xfc format).
7. Now find the new image in the folder where you saved it, and drag it to the panel. Right click on the panel, go to properties and set its height to 37 pixels. You'll see that the panel now looks thinner, and the icons seem to be higher than the panel!
Of course, you can play with these steps until you get the desired look. The numbers 20 and 37 that I'm using here can be modified until you are happy with the results.
I'm attaching a file with two examples you can try.
(And if anyone knows a better word for "protuberant", please let me know)