Why does LO5.1.4 Writer "PDF export" don't compress PNG images if JPEG compression is enabled?

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PNG files are already compressed and cannot be compressed further.

1. ALWAYS use JPG files for photos.
2. NEVER copy and paste a photo into a document as LO does not know what format the original image was in and will (correctly) save it as a PNG to ensure no data loss.
3. ALWAYS add photos by Insert > Image ...
4. NEVER use PNG for photos as a PNG file will typically be 3x bigger than the JPG
5. ALWAYS use PNG (or TIF or GIF) for graphics (ie not photos - things like clip art, diagrams, anything with text etc).
6. NEVER use JPG for graphics as it blurs edges and makes text very poor.
7. NEVER use BMP as it is uncompressed.
8. A 6 million pixel image from a camera is typically 3,200 pixels wide which is usually far too many pixels for for most purposes. Decide what resolution you require for the final document. If the image will be, say, 3 inches wide on paper and you are printing at 150 dpi (very good quality) or 300 dpi (excellent quality) or 600 dpi (superb quality) the the image needs to be 3 x 150 = 450 pixels wide for 150 dpi printing; 3 x 300 = 900 pixels wide for 300 dpi; and 3 x 600 = 1800 pixels wide for 600 dpi printing. If the image will be viewed on screen, what is the screen resolution? A high definition 15.6" wide laptop screen typically has 1,366 pixels - there is no point in your image being higher resolutions than this.
9. Do all image processing in an image editor before placing the image in the document. LO is not a good image processor when compared with proper image processors. Resample images to reduced resolutions with a graphics editor like the free IrfanView (simple to use) or GIMP (more powerful and much more difficult to use).
10. JPG files use lossy compression where data is lost. PNG files use lossless compression where no data is lost. Google JPG lossy compression and PNG lossless compression for more information.