diff --git a/Tests/PhotochkTests/TestImages.swift b/Tests/PhotochkTests/TestImages.swift
new file mode 100644
index 0000000..6a13e63
--- /dev/null
+++ b/Tests/PhotochkTests/TestImages.swift
@@ -0,0 +1,129 @@
+import CoreGraphics
+import Foundation
+import ImageIO
+import UniformTypeIdentifiers
+
+/// テスト用の画像を決まった内容で作る。乱数は自前の線形合同法なので毎回同じ絵になる。
+enum TestImages {
+    static let width = 640
+    static let height = 480
+
+    struct Generator {
+        private var state: UInt64
+
+        init(seed: UInt64) { state = seed &* 2_862_933_555_777_941_757 &+ 3_037_000_493 }
+
+        mutating func next() -> UInt64 {
+            state = state &* 6_364_136_223_846_793_005 &+ 1_442_695_040_888_963_407
+            return state
+        }
+
+        mutating func unit() -> Double {
+            Double(next() >> 11) / Double(1 << 53)
+        }
+    }
+
+    /// 色のついたブロックと円を敷き詰めた、構造のある絵を作る。
+    static func pattern(seed: UInt64) -> CGImage {
+        var generator = Generator(seed: seed)
+
+        let context = CGContext(
+            data: nil,
+            width: width,
+            height: height,
+            bitsPerComponent: 8,
+            bytesPerRow: 0,
+            space: CGColorSpaceCreateDeviceRGB(),
+            bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue
+        )!
+
+        let columns = 8
+        let rows = 6
+        let cellWidth = CGFloat(width) / CGFloat(columns)
+        let cellHeight = CGFloat(height) / CGFloat(rows)
+
+        for row in 0..<rows {
+            for column in 0..<columns {
+                context.setFillColor(
+                    red: generator.unit(),
+                    green: generator.unit(),
+                    blue: generator.unit(),
+                    alpha: 1
+                )
+                context.fill(
+                    CGRect(
+                        x: CGFloat(column) * cellWidth,
+                        y: CGFloat(row) * cellHeight,
+                        width: cellWidth,
+                        height: cellHeight
+                    )
+                )
+            }
+        }
+
+        for _ in 0..<12 {
+            context.setFillColor(
+                red: generator.unit(),
+                green: generator.unit(),
+                blue: generator.unit(),
+                alpha: 0.85
+            )
+            let radius = 20 + generator.unit() * 60
+            context.fillEllipse(
+                in: CGRect(
+                    x: generator.unit() * Double(width),
+                    y: generator.unit() * Double(height),
+                    width: radius,
+                    height: radius
+                )
+            )
+        }
+
+        return context.makeImage()!
+    }
+
+    static func scaled(_ image: CGImage, factor: Double) -> CGImage {
+        let newWidth = max(1, Int(Double(image.width) * factor))
+        let newHeight = max(1, Int(Double(image.height) * factor))
+
+        let context = CGContext(
+            data: nil,
+            width: newWidth,
+            height: newHeight,
+            bitsPerComponent: 8,
+            bytesPerRow: 0,
+            space: CGColorSpaceCreateDeviceRGB(),
+            bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue
+        )!
+        context.interpolationQuality = .high
+        context.draw(image, in: CGRect(x: 0, y: 0, width: newWidth, height: newHeight))
+        return context.makeImage()!
+    }
+
+    static func writeJPEG(_ image: CGImage, to url: URL, quality: Double) throws {
+        guard let destination = CGImageDestinationCreateWithURL(
+            url as CFURL, UTType.jpeg.identifier as CFString, 1, nil
+        ) else {
+            throw TestImageError.cannotWrite(url)
+        }
+        CGImageDestinationAddImage(
+            destination,
+            image,
+            [kCGImageDestinationLossyCompressionQuality: quality] as CFDictionary
+        )
+        guard CGImageDestinationFinalize(destination) else {
+            throw TestImageError.cannotWrite(url)
+        }
+    }
+
+    static func makeTemporaryDirectory() throws -> URL {
+        let url = URL(fileURLWithPath: NSTemporaryDirectory())
+            .appendingPathComponent("PhotochkTests-\(UUID().uuidString)", isDirectory: true)
+        try FileManager.default.createDirectory(at: url, withIntermediateDirectories: true)
+        return url
+    }
+}
+
+enum TestImageError: Error {
+    case cannotWrite(URL)
+}