copyExternalImageToTexture has a fixed set of allowed target formats:

• "r8unorm"
• "r16float"
• "r32float"
• "rg8unorm"
• "rg16float"
• "rg32float"
• "rgba8unorm"
• "rgba8unorm-srgb"
• "bgra8unorm"
• "bgra8unorm-srgb"
• "rgb10a2unorm"
• "rgba16float"
• "rgba32float"

We recently added 16unorm formats to the spec in (#5160) but didn't update this list. We should, for 16-bit PNGs for example.