If you are correctly mentioning the file name in "GNUmakefile" as a resource (and hence it gets copied inside the .app), try instantiating the image using method +[NSImage imageNamed:].
NSImage *myImage = [NSImage imageNamed:@"imagen"]; // autoreleased
Optionally retain it; the object returned is autoreleased.
Alternatively get the path like this:
NSString *path = [[NSBundle mainBundle] pathForResource:@"imagen" ofType:@"png"];
NSImage *myImage = [[NSImage alloc] initWithContentsOfFile:path]; // retained already
It's a bad idea to reference to things using an absolute path. Always try to refer to paths using some API, even when referring to paths inside the app bundle. When you move the app to a new platform, you'll be glad you did.
If nothing of the above helps, try using a different .png to verify your GNUstep installation is capable of decoding the PNG file format.