How do I export UIImage array as a movie?

Take a look at AVAssetWriter and the rest of the AVFoundation framework. The writer has an input of type AVAssetWriterInput, which in turn has a method called appendSampleBuffer: that lets you add individual frames to a video stream. Essentially you’ll have to:

1) Wire the writer:

NSError *error = nil;
AVAssetWriter *videoWriter = [[AVAssetWriter alloc] initWithURL:
    [NSURL fileURLWithPath:somePath] fileType:AVFileTypeQuickTimeMovie
    error:&error];
NSParameterAssert(videoWriter);

NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
    AVVideoCodecH264, AVVideoCodecKey,
    [NSNumber numberWithInt:640], AVVideoWidthKey,
    [NSNumber numberWithInt:480], AVVideoHeightKey,
    nil];
AVAssetWriterInput* writerInput = [[AVAssetWriterInput
    assetWriterInputWithMediaType:AVMediaTypeVideo
    outputSettings:videoSettings] retain];

NSParameterAssert(writerInput);
NSParameterAssert([videoWriter canAddInput:writerInput]);
[videoWriter addInput:writerInput];

2) Start a session:

[videoWriter startWriting];
[videoWriter startSessionAtSourceTime:…]

3) Write some samples:

// Or you can use AVAssetWriterInputPixelBufferAdaptor.
// That lets you feed the writer input data from a CVPixelBuffer
// that’s quite easy to create from a CGImage.
[writerInput appendSampleBuffer:sampleBuffer];

4) Finish the session:

[writerInput markAsFinished];
[videoWriter endSessionAtSourceTime:…];
[videoWriter finishWriting];

You’ll still have to fill-in a lot of blanks, but I think that the only really hard remaining part is getting a pixel buffer from a CGImage:

- (CVPixelBufferRef) newPixelBufferFromCGImage: (CGImageRef) image
{
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
        [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey,
        [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey,
        nil];
    CVPixelBufferRef pxbuffer = NULL;
    CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, frameSize.width,
        frameSize.height, kCVPixelFormatType_32ARGB, (CFDictionaryRef) options, 
        &pxbuffer);
    NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL);

    CVPixelBufferLockBaseAddress(pxbuffer, 0);
    void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer);
    NSParameterAssert(pxdata != NULL);

    CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(pxdata, frameSize.width,
        frameSize.height, 8, 4*frameSize.width, rgbColorSpace, 
        kCGImageAlphaNoneSkipFirst);
    NSParameterAssert(context);
    CGContextConcatCTM(context, frameTransform);
    CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image), 
        CGImageGetHeight(image)), image);
    CGColorSpaceRelease(rgbColorSpace);
    CGContextRelease(context);

    CVPixelBufferUnlockBaseAddress(pxbuffer, 0);

    return pxbuffer;
}

frameSize is a CGSize describing your target frame size and frameTransform is a CGAffineTransform that lets you transform the images when you draw them into frames.