After deleting/adding some png files to project, i have got messages when building project.
\"file ProjectPath\\aaa\\xxx.png is missing from working copy.\"
To add onto Alexander Vasenin's answer...
First I Committed and Pushed my changes
Xcode Main Menu > Source Control > Commit
Then I Discarded All Changes to get rid of the errors
Xcode Main Menu > Source Control > Discard All Changes
After that, the errors stating "file xxx.png is missing from working copy" disappeared.