The Xcode license and your developer agreement are the two documents you should read carefully to be certain you understand your obligations, but Xcode is delivered to developers for the express purpose of making apps.
Clearly, the graphic elements like buttons and standard API are permitted use - royalty free per the licensing term. This is covered in the Xcode agreement which on some versions reads like this:
F. Use of System-Provided Images. The system-provided images owned by Apple and documented as such in Apple's Human Interface Guidelines for OS X and iOS ("System-Provided Images") are licensed to you solely for the purpose of developing OS X and iOS applications, respectively, using the Developer Software. Upon termination of this License, you may continue to distribute the System-Provided Images as used within software you developed using the Developer Software; however, you agree to provide to Apple, at Apple's request, a copy of any software you developed using the Developer Software that incorporates any of the System-Provided Images so Apple may determine, in its sole discretion, whether your use of the System-Provided Images complies with these terms.
I've seen all sorts of programs use default sprites and effects, but you might want to consider paying someone for an alternate image if you are really uncomfortable with your understanding of the rules you agreed to to develop software using Apple's tools. To me, it seems that Apple would have paid for the rights to that artwork and you don't need to further compensate that artist as long as you are abiding by the agreements you signed with Apple to use Xcode.
If I were doing this, I would re-read the entire agreement I signed before sending that app to the App store - for many reasons beyond just this one image. If you are paying to be a Mac developer, you can certainly ask developer support to be clear that they understand your intentions and you have something in writing before shipping the app.
It really depends on several factors:
- Will you then use this in marketing materials or does it just show up in the app?
- Is it your app's Home Screen icon and store icon?
- Are you going through App Review and only distributing your app through Apple?
- What's the budget for your entire development?
- Do you have a lawyer that you pay to keep you out of trouble?