在Swift中，我们可以使用AVFoundation框架来生成图像缩略图。以下是示例代码：

import AVFoundation
import UIKit

func generateThumbnail(url: URL) -> UIImage? {
    do {
        let asset = AVURLAsset(url: url)
        let imgGenerator = AVAssetImageGenerator(asset: asset)
        imgGenerator.appliesPreferredTrackTransform = true
        let cgImage = try imgGenerator.copyCGImage(at: CMTimeMake(value: 0, timescale: 1), actualTime: nil)
        let uiImage = UIImage(cgImage: cgImage)
        return uiImage
    } catch let error {
        print("*** Error generating thumbnail: \(error.localizedDescription)")
        return nil
    }
}

// Usage: 
let url = URL(fileURLWithPath: "path_to_local_video.mov")
let thumbnail = generateThumbnail(url: url)
if let image = thumbnail {
    // Do something with the thumbnail image
}

该方法会生成本地视频的第一帧图像作为缩略图，并返回UIImage对象。您可以在生成缩略图后根据需要进行操作。