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One of the easiest ways to spruce up your program is to add an application icon. To do this, create a 128x128 tiff image with alpha channel information (for the transparent parts). If you aren't able to afford Adobe's Photoshop, I suggest GimpShop, a version of the open-source Gimp graphic editor that is Mac-specific and has been revised to be more "Photoshop-like." In like manner, Inkscape is an open-source vector drawing program that has much of the functionality of Adobe's Illustrator.
While XCode can use your tiff image 'as is', the Apple Developer Tools provide a method to convert your tiff into an actual icon file (type icns). The Icon Composer application is very straightforward. When you launch the program, it opens a window with four tiers, one for each static size (other sizes are interpolated). At the bottom is the largest size, 128x128, which is where you will put your tiff file. You can simply drag your tiff file into the left-hand box (or use the Import Image menu item in the File menu). If your image is larger than 128x128, you will be asked if you want to use a scaled version of the tiff.
Once the thumbnail size (128) is loaded, you can drag that image into the other sizes' left-hand boxes and let the program scale them and compute the icon mask. If you don't like the way the program scales them, you can create your own versions at the various sizes(16, 32, and 48) and import them. If you look carefully at some of the icons on your Mac, you'll notice that at the smaller sizes sometimes the icon creator uses a closeup of the original graphic in order to get enough detail in the smaller icon to make it clear, rather than just scaling the original image.
If you want to see your icon gracing your completed icon file, a simple utility I found called icns2icon will take your new icon and transfer it to the file you just created. Just drag your icon file onto the icns2icon application. Now you can see your icons at a glance.
To install your application icon into your project, right click on the resources folder in XCode's project view and select Add then Existing Files... When XCode asks if you want to copy the file into your project directory, you can choose to do so if you like, or leave it where it is now and just add a reference to it. Either way, you now need to inform your project that this icon is to be used for the application.
To do this, select the Edit Active Target 'yourappname' item from the Project menu. Go to the Properties tab and put the name of your icon into the supplied form, where it says "Icon File."
Now when you build your project, you'll have your own custom icon!