pw_presubmit: ignore images in copyright check
pw_presubmit: ignore images in copyright check

Adds image file extensions to the ignore list when checking copyright
notice.

Change-Id: Ia78d92764e396f48bf42a488e357bf24be4adbaf

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed