What counts as an image
Photographs, scans and logos placed in the PDF as pictures. Charts and diagrams drawn as vector shapes are not images, so there is nothing to extract from them — use PDF to PNG to turn the whole page into a picture instead.
What is skipped
Images smaller than 16 pixels on a side, which are nearly always bullets or rules, and one-colour stencil masks, which are shapes the PDF paints through rather than pictures. An image used on several pages is saved once.
Why PNG
The tool gets the decoded pixels, not the original compressed bytes, and PNG stores them without any further loss. A photo that was a JPEG inside the PDF will therefore give a larger file than the original.