add test image readme

test_images/README.md

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+# Test Image
+The test image was generated by DALL-E in ChatGPT-4, with the below prompt:
+
+```
+A picturesque landscape featuring a vibrant array of colors. The scene includes a lush green meadow under a bright blue sky, dotted with fluffy white clouds. In the background, majestic mountains are visible, with peaks partially covered in snow. A crystal-clear river meanders through the meadow, reflecting the sky and surrounding nature. The scene is illuminated by a warm, golden sunlight, creating a serene and beautiful atmosphere perfect for testing image conversion software.
+```