Makefile - Add To The Bottom of your Makefile and change NameOfYourBundle to be the name you wish.
NOTE: It cannot be the same as your TWEAK_NAME
BUNDLE_NAME = NameOfYourBundle
NameOfYourBundle_INSTALL_PATH = /Library/MobileSubstrate/DynamicLibraries
include $(THEOS)/makefiles/bundle.mk
Resources - In the main directory of your tweak add a folder called Resources and place your images inside it.
Code - Create a NSBundle, init with "initWithPath:", get the image path with "pathForResource:", get image with "imageWithContentsOfFile:"
#define kBundlePath @"/Library/MobileSubstrate/DynamicLibraries/NameOfYourBundle.bundle"
//Get the Bundle
NSBundle *bundle = [[NSBundle alloc] initWithPath:kBundlePath];//Get Path to Image
NSString *imagePath = [bundle pathForResource:@"imageName" ofType:@"png"];
//UIImage From File
UIImage *image = [UIImage imageWithContentsOfFile:imageOnePath];
NOTE:The same basic idea also works for other files types, just use the appropriate classes equivalent method