Your problem is probably that the image is not copied to the target. In xcode groups & file pane, fine the node for targets, expand your app target and you'll see Copy Bundle resources expand it and you'll see all the files copied to the default resource bundle when you deploy your app to the sim/device. Only the resources found there will be available to your app. If you don't see your image there you can just drag it there and everything will work.
Now, in order to understand why this might happened we need to look at the different options for adding files to xcode.
When you're adding a file to xcode you are presented with a dialog that includes a few interesting and sometimes overlooked option.
The first - Should the file be copied to the project folder, or should it just point to the file original location. The safest way will be to always copy the files to the project's path. However for images (such in your case) I do it differently. I maintain a separate graphics folder for every app, I save images there (both sources, and final png). When I add an image I do not copy it to the project folder, rather xcode just points to its location. That way I can just edit the image in photoshop, do a rebuild, and the updated image will show up on the phone. Whats important to understand that the directory structure of files you include in the project have almost no meaning, files that appear in the Copy Bundle Resources for the target will always be copied to a single (and flat) bundle/"directory" on the phone. The only exception for this is if you actually add directories to xcode (Personally I recommend against it).
The second option when adding files, is to which targets to add the file. This is the list with checkboxes on the bottom, if when you added the image, your app target wasn't checked the file won't be included in the target Copy bundle resource, and won't be available for the phone.
A few caveats - When using references to files in other location, that location is relative to the project's path, so it's a good idea to keep all the files in directories below the project's root directory. That way you can still copy the entire project to a different location and it will still work.