Since ImageMagick doesn’t support components with arbitrary bit depth (only 8/16/32/etc), the simplest solution is to convert the UIImage into a consistent format before processing with ImageMagick.

Here is something to do just that (also fixed a leaking data provider):

CGImageRef createStandardImage(CGImageRef image) {
	const size_t width = CGImageGetWidth(image);
	const size_t height = CGImageGetHeight(image);
	CGColorSpaceRef space = CGColorSpaceCreateDeviceRGB();
	CGContextRef ctx = CGBitmapContextCreate(NULL, width, height, 8, 4*width, space,
											 kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedFirst);
	CGColorSpaceRelease(space);
	CGContextDrawImage(ctx, CGRectMake(0, 0, width, height), image);
	CGImageRef dstImage = CGBitmapContextCreateImage(ctx);
	CGContextRelease(ctx);
	return dstImage;
}

- (UIImage*) createPosterizeImage:(CGImageRef)srcCGImage {
	const unsigned long width = CGImageGetWidth(srcCGImage);
	const unsigned long height = CGImageGetHeight(srcCGImage);

	// could use the image directly if it has 8/16 bits per component,
	// otherwise the image must be converted into something more common (such as images with 5-bits per component)
	// here we'll be simple and always convert
	const char *map = "ARGB"; // hard coded
	const StorageType inputStorage = CharPixel;
	CGImageRef standardized = createStandardImage(srcCGImage);
	NSData *srcData = (NSData *) CGDataProviderCopyData(CGImageGetDataProvider(standardized));
	CGImageRelease(standardized);

	const void *bytes = [srcData bytes];
	MagickWandGenesis();
	MagickWand *magick_wand = NewMagickWand();

	MagickBooleanType status = MagickConstituteImage(magick_wand, width, height, map, inputStorage, bytes);
	if (status == MagickFalse) {
		ThrowWandException(magick_wand);
	}

	status = MagickOrderedPosterizeImage(magick_wand, "h8x8o");
	if (status == MagickFalse) {
		ThrowWandException(magick_wand);
	}

	const int bitmapBytesPerRow = (width * strlen(map));
	const int bitmapByteCount = (bitmapBytesPerRow * height);
	CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
	char *trgt_image = malloc(bitmapByteCount);
	status = MagickExportImagePixels(magick_wand, 0, 0, width, height, map, CharPixel, trgt_image);
	if (status == MagickFalse) {
		ThrowWandException(magick_wand);
	}

	magick_wand = DestroyMagickWand(magick_wand);
	MagickWandTerminus();

	CGContextRef context = CGBitmapContextCreate (trgt_image,
												  width,
												  height,
												  8, // bits per component
												  bitmapBytesPerRow,
												  colorSpace,
												  kCGImageAlphaPremultipliedFirst);
	CGColorSpaceRelease(colorSpace);

	CGImageRef cgimage = CGBitmapContextCreateImage(context);
	UIImage *image = [[UIImage alloc] initWithCGImage:cgimage];
	CGImageRelease(cgimage);
	CGContextRelease(context);
	[srcData release];
	free(trgt_image);

	return image;
}

thanks a lot for guidance
how can i store UIImage into custom location and process the file using ImageMagick.