How did you add the image? Copy and paste? If so, Agenda does not get the original file, but instead gets just an image in memory. It doesn’t know the original file, so it saves a PNG of the memory contents.
We use PNG because it can handle transparency, and JPEG can’t. That doesn’t matter for you, but for others, who have a transparent image, it could be important.
We will look at whether we can perhaps drop to using JPEG instead of PNG in some cases, eg, when there is no transparency.
Note that if you use insert photo or attach file, I believe you should get the original image inserted. I think it is only copy/paste and drag/drop that can lead to a different image format.