I am having trouble believing this. Please demonstrate by sending a message including such a picture to me at xenos@gmx.co.uk. Thank you.

Thanks. I have received your test email and have viewed it in three different clients on this Android tablet (I've just put away my proper computer for the night) and all three correctly show the attached picture of a Sandisk SD Card. The two that actually provide information about the attachment report it to be a jpeg file. I'll look at it again tomorrow when I have Thunderbird and maybe another email client to hand.

I was doubtful about your reported "pjp" filetype simply because I can't find a definitive description that tells me what application would open it.

If there was any image conversion going on, I'd have expected png to be used. When you add an image or a file as an attachment, it's just a set of bytes and should be encoded "as is"; no email client could claim to be able to parse just any arbitrary filetype it is given, so needs to do its best to convey the attached file unchanged, verbatim. The snag is that email is a purely text medium so anything that isn't text needs to be encoded in a way that represents binary data using printable characters. One common encoding system is "base-64", but there are others.

An embedded picture is another matter. A load of bytes is inserted into your message and to some extent the email client is free to encode them in whatever way it wants. This is where I might expect png encoding to be used. Unlike gif, it isn't limited to a restricted palette, and unlike jpeg, it is capable of lossless encoding. But I have never heard of pjp and I am not seeing any sign of it being used here in your email message.