Created attachment 178031 [details]
A sample JPEG from https://upload.wikimedia.org/wikipedia/commons/thumb/6/62/Henry_Taube_-_HD.3F.005_%2811086397086%29.jpg/350px-Henry_Taube_-_HD.3F.005_%2811086397086%29.jpg
1. Open https://upload.wikimedia.org/wikipedia/commons/thumb/6/62/Henry_Taube_-_HD.3F.005_%2811086397086%29.jpg/350px-Henry_Taube_-_HD.3F.005_%2811086397086%29.jpg in Firefox (I used 96.0.3 (64-bit) on Windows 10).
2. Drag the image from the browser into a Writer document.
3. Save the document as ODT.
4. Inspect the ODT (zip package) to see that the image is stored in /Pictures as a ~350 KB BMP, not ~38 KB JPEG.
If you download the JPEG locally, and then drag it from Windows Explorer to Writer document, it will be stored as JPEG.
This problem is mentioned on https://forum.openoffice.org/en/forum/viewtopic.php?f=71&t=86682 (see #13), so a longstanding problem. Possibly related to how FF works with Windows clipboard; in any case - even if the image format is unknown - it would possibly make sense to prefer PNG over BMP (using internal Compress, and defining compression level 9 without downsampling, gives smaller image and ODT file size, and also faster operation - possibly because of less disk IO).
Looks caused by Firefox bug https://bugzilla.mozilla.org/show_bug.cgi?id=557708, and thus NOTOURBUG (since the actual file being inserted into Writer is coming from Firefox saved temporary copy like C:\Users\user\AppData\Local\Temp\dvlz7uhh.bmp), and has the BMP "original" format, which is rightfully kept when stored.