Pasted images can arrive as size-only placeholders; use a file path

In a long Claude Code session, three images pasted by the user arrived as only '[Image: original WxH, displayed at …]' with no image content. Nothing was saved to the session transcript (no image block in the .jsonl), and the macOS clipboard held only text afterwards, so the picture was unrecoverable. Ask the user for a file instead: check defaults read com.apple.screencapture location (here ~/Downloads), have them take a normal ⌘⇧4 screenshot, and watch that folder for Screenshot*.png. Then Read the file. Don't guess at what the image showed.