Normally you are adding Icons through the app xml file.
<icon> <image16x16>icons/icon16x16.png</image16x16> <image32x32>icons/icon32x32.png</image32x32> <image48x48>icons/icon48x48.png</image48x48> <image128x128>icons/icon128x128.png</image128x128> </icon>
To avoid errors, the following should be considered: Copy your Icons-Folder (in my case named ‘icons’) inside the ‘src’-folder, select the Project->Properties->FlexCompiler Option ‘Copy non-embed files to output folder’ and restart the release build. Thats it.