I created artifacts/image.png: a 512 × 512 PNG showing a flat pink (#ec4f9a) square centered on a solid #0e0e12 background.

• Size of the square: it is 396 × 396 pixels, running from pixel 58 to 453 on each axis. That covers about 59.8% of the canvas area. I took "about 60% of the canvas" to mean area; if you meant 60% of the width, the square would be about 307 pixels wide instead.
• Flat color: I decoded the file and it contains exactly two colors, so there are no gradients, edge smoothing, border, shadow, text or other shapes.
• Checks: the header reads 512 × 512, 8-bit RGB. The pixels on either side of each edge of the square are the right colors, and the image looked correct when I opened it. ffprobe isn't installed here, so I did these checks with Python and file instead.
• Unmet requirements: none that I know of.

I also wrote artifacts/README.md, which records the size, format, how the image was made and its limits. Both files are left untracked, as instructed. The script that generates the image is in test/scratch/gen.py, which isn't submitted.