Pasting images from clipboard is a very poor way to add images. It depends very much on the application it is copied from, some might put only a bitmap image on the clipboard and then Writer will convert it to png but of course .bmp does not support transparency. Other apps like the GIMP will put png file on clipboard. If it is a jpg that is copied then Writer will convert the image to png increasing file size but keeping the jpg artefacts. Pasted images are always saved as png in Writer.
The best way to place images is to
- Click Insert > Image. Or
- Drag image from a file manager such as Explorer
Both those add the original file to Writer so the image retains the benefits of that file type.