The memory-efficient way of using Core Image on iOS?

I'm using Core Image filters in my app, everything works fine on my iPhone 5 device running iOS 7, but when I test it on iPhone 4s, which has only a total memory of 512MB, the app crashes.

Here's the situation, I have 2 images taken from the camera, with a resolution of 2448x3264 each. In my iPhone 5, the whole process takes up 150MB at the peak according to instruments.

However, when I try to run the same code on iPhone 4s, the instruments gave me memory low warning all the time, even if the whole memory use is quite low (around 8 MB). Here's the screenshot below.

And here's the code, basically, I loaded two images from documents folder of my app, and applied 2 filters in a row:

    CIImage *foreground = [[CIImage alloc] initWithContentsOfURL:foregroundURL];
    CIImage *background = [[CIImage alloc] initWithContentsOfURL:backgroundURL];
    CIFilter *softLightBlendFilter = [CIFilter filterWithName:@"CISoftLightBlendMode"];
    [softLightBlendFilter setDefaults];
    [softLightBlendFilter setValue:foreground forKey:kCIInputImageKey];
    [softLightBlendFilter setValue:background forKey:kCIInputBackgroundImageKey];

    foreground = [softLightBlendFilter outputImage];
    background = nil;
    softLightBlendFilter = nil;

    CIFilter *gammaAdjustFilter = [CIFilter filterWithName:@"CIGammaAdjust"];
    [gammaAdjustFilter setDefaults];
    [gammaAdjustFilter setValue:foreground forKey:kCIInputImageKey];
    [gammaAdjustFilter setValue:[NSNumber numberWithFloat:value] forKey:@"inputPower"];
    foreground = [gammaAdjustFilter valueForKey:kCIOutputImageKey];

    gammaAdjustFilter = nil;

    CIContext *context = [CIContext contextWithOptions:nil];
    CGRect extent = [foreground extent];
    CGImageRef cgImage = [context createCGImage:foreground fromRect:extent];

    UIImage *image = [UIImage imageWithCGImage:cgImage scale:1.0 orientation:imgOrientation];
    CFRelease(cgImage);
    foreground = nil;

    return image;

The app crashed at this line: CGImageRef cgImage = [context createCGImage:foreground fromRect:extent];

Is there any more memory-efficient way of handling this situation, or what am I doing wrong here?

Big thanks!