Using Charles 4.1.3:
- Right-click request in Structure pane
- Select "Export..."
- Select Format "HTTP Trace File (.trace)", name the file
- Open the file in a text editor, such as TextMate. When asked for encoding, Western - ISO Latin 1 is just fine
- Remove all text from the start that contains header info, up until and including the line that says something like "Request-Body:<<--EOF-1498740958503-"
- Scroll all the way down, and remove all text starting at the line that looks like this: "--EOF-1498741139055-"
- Save
- Preview will already be an image. When renamed to the actual format, more information, such as image size, will become available