How to render a video file of fixed length from a still image (png)

I am trying to create a video of a specified length from a still image (png).
My code is based on the answers given in How do I export UIImage as a movie

My code creates a video of the correct length, but the image does appear until the last frame. I want the immage to be shown for the full duration of the video. (These videos will be place-holders in a gapless timeline.)

My Code Is:

- (NSURL*)exportImage:(NSImage*)image asMovieLength:(double)seconds {

	NSURL *outFile;
	AppDelegate *theApp;
	NSError *error=nil;
	AVAssetWriter *videoWriter;
	NSDictionary *videoSettings;
	AVAssetWriterInput* writerInput;
	CVPixelBufferRef buffer = NULL;
	AVAssetWriterInputPixelBufferAdaptor *adaptor;
	
	theApp = (AppDelegate*)[[NSApplication sharedApplication] delegate];
	
	outFile = [theApp getFilenameForFillerFile];
	
	videoWriter = [[AVAssetWriter alloc] initWithURL:outFile fileType:AVFileTypeMPEG4 error:&error];
	NSParameterAssert(videoWriter);

	videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:AVVideoCodecH264, AVVideoCodecKey,
					 [NSNumber numberWithInt:880], AVVideoWidthKey,
					 [NSNumber numberWithInt:880], AVVideoHeightKey,
					 nil];
	
	writerInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
	
	adaptor = [AVAssetWriterInputPixelBufferAdaptor assetWriterInputPixelBufferAdaptorWithAssetWriterInput:writerInput
																			   sourcePixelBufferAttributes:nil];
	
	NSParameterAssert(writerInput);
	NSParameterAssert([videoWriter canAddInput:writerInput]);
	[videoWriter addInput:writerInput];
	
	// Start the writing session
	[videoWriter startWriting];
	[videoWriter startSessionAtSourceTime:kCMTimeZero];
	
	
	if(writerInput.readyForMoreMediaData)
	{
		CMTime zeroTime = CMTimeMake(0, 600);
		CMTime frameTime = CMTimeMake(seconds*600, 600);
		buffer = [self pixelBufferFromImage:image];

		[adaptor appendPixelBuffer:buffer withPresentationTime:frameTime];

		[writerInput markAsFinished];
		
		[videoWriter finishWritingWithCompletionHandler:^{
			NSLog(@"Finished writing...checking completion status...");
			if (videoWriter.status != AVAssetWriterStatusFailed && videoWriter.status == AVAssetWriterStatusCompleted)
			{
				NSLog(@"Video writing succeeded.");
				
			} else
			{
				NSLog(@"Video writing failed: %@", videoWriter.error);
			}
			
		}];
		
		CVPixelBufferPoolRelease(adaptor.pixelBufferPool);
		
		NSLog (@"Done");
	}
	
	return outFile;
}

How do I get the image to show from the start, and last for the specified time?

Following changes in your code did the trick for me.

CMTime zeroTime = CMTimeMake(0, 600);
CMTime frameTime = CMTimeMake((seconds * 0.5) * 600, 600);

buffer = [self pixelBufferFromImage:image];
        
[adaptor appendPixelBuffer:buffer withPresentationTime:zeroTime];
[adaptor appendPixelBuffer:buffer withPresentationTime:frameTime];