That is not the way you include resources using theos. MyTweak_FILES variable should only include files that can be compiled. Make file handles resources differently.
To include resources you need to create a bundle as follows.
1) Create a folder called Resources in the tweak.xm directory.
2) Put all your resource files (all your PNG's) into that folder.
3) Add the following info to your make file
BUNDLE_NAME = your_bundle_identifier
your_bundle_identifier_INSTALL_PATH = /Library/MobileSubstrate/DynamicLibraries
4) Define your bundle as follows on top of your tweak.xm file.
#define kBundlePath @"/Library/MobileSubstrate/DynamicLibraries/your_bundle_identifier.bundle"
5) You can now initialize the bundle and use the images within your tweak as follows:
NSBundle *bundle = [[[NSBundle alloc] initWithPath:kBundlePath] autorelease];
NSString *imagePath = [bundle pathForResource:@"your_image_name" ofType:@"png"];
UIImage *myImage = [UIImage imageWithContentsOfFile:imagePath]
In the above steps replace your_bundle_identifier with your tweaks bundle identifier which would be in the control file. (ex: com.yourdomain.tweak_name)
Also replace your_image_name with the name of the image you want to use.
You can pretty much use any resources (ex: sound files) the above way.