Walgreens photo uploaded would only let me crop, but I had a bunch of square photos, so...

aspect-grow.swift

#!/usr/bin/swift

// Because sometimes you don't want to crop.

// USAGE: aspect-grow RATIO FILES...
//        aspect-grow 4:6 *.jpg
//
// Places all files in an output directory relative to their location, so
// aspect-grow 4:6 *.jpg creates a folder "output" in this directory but
// aspect-grow 4:6 a/b.jpg and b/c.jpg creates folders a/output and b/output.

import Cocoa

struct AspectRatio : CustomStringConvertible {
    var width:CGFloat
    var height:CGFloat
    
    init?(width:CGFloat, height:CGFloat) {
        
        if width <= 0 || height <= 0 {
            return nil
        }
        
        self.width = width
        self.height = height
    }
    
    init?(_ string:String) {
        
        let parts = string.componentsSeparatedByString(":")
        if parts.count != 2 {
            return nil
        }
        
        guard let width = Double(parts[0]), height = Double(parts[1]) else {
            return nil
        }
        
        self.init(width: CGFloat(width), height: CGFloat(height))
    }
    
    var description:String {
        return "\(width):\(height)"
    }
    
    func rotated() -> AspectRatio {
        return AspectRatio(width: height, height: width)!
    }
}

extension CGSize {
    mutating func grow(aspectRatio:AspectRatio) {
        if width < height * aspectRatio.width / aspectRatio.height {
            width = height * aspectRatio.width / aspectRatio.height
        } else {
            height = width * aspectRatio.height / aspectRatio.width
        }
    }
    
    func grown(aspectRatio:AspectRatio) -> CGSize {
        var size = self
        size.grow(aspectRatio)
        return size
    }
    
    var diagonal:CGFloat {
        return sqrt(width * width + height * height)
    }
    
    func rotated() -> CGSize {
        return CGSize(width: height, height: width)
    }
}

func bestSizeForImageAtPath(path:String, aspectRatio:AspectRatio) -> CGSize? {
    
    guard let size = NSImage(contentsOfFile: path)?.size else {
        return nil
    }
    
    var normalSize = size.grown(aspectRatio)
    var rotatedSize = size.grown(aspectRatio.rotated())
    
    if normalSize.diagonal < rotatedSize.diagonal {
        return normalSize
    } else {
        return rotatedSize
    }
}

func resizeImageAtPath(path:String, aspectRatio:AspectRatio) {
    
    guard let newSize = bestSizeForImageAtPath(path, aspectRatio: aspectRatio) else {
        preconditionFailure("Could not determine aspect ratio for file at \(path)")
    }

    do {
        try NSFileManager().createDirectoryAtPath(path.stringByDeletingLastPathComponent.stringByAppendingPathComponent("output"), withIntermediateDirectories: false, attributes: nil)
    } catch {
    }
    
    let task = NSTask()
    task.launchPath = "/usr/local/bin/convert"
    task.arguments = [
        "-size", "\(Int(newSize.width))x\(Int(newSize.height))",
        "xc:white",
        path,
        "-gravity", "center",
        "-composite",
        path.stringByDeletingLastPathComponent.stringByAppendingPathComponent("output").stringByAppendingPathComponent(path.lastPathComponent)
    ]
    
    task.launch()
    task.waitUntilExit()

    if task.terminationStatus != 0 {
        preconditionFailure("ImageMagick failed with arguments: \(task.arguments!)")
    }
}

let args = Process.arguments

precondition(args.count >= 3, "USAGE: aspect-grow RATIO FILES...")

guard let aspectRatio = AspectRatio(args[1]) else {
    preconditionFailure("Aspect ratio must be of format A:B, e.g. 4:6")
}

for path in args[2..<args.endIndex] {
    resizeImageAtPath(path, aspectRatio: aspectRatio)
}