I'll probably post this in 'Ideas' as well, but as I find myself wondering why I seem to be totally alone in still being slightly open-mouthed at Thunderbird's 'method' for inserting images, I thought I'd try 'Discussions' as well.
Quite why Thunderbird doesn't have a native, reasonably intuitive, and most of all sensible way of inserting an image into the body text of an email I've no idea. I've been following the email discussions on beta development/testing of version 115, and although I'm full of admiration and gratitude for all the hard work, and hours of work that goes into these updates... - Thunderbird still can't just 'include a resized image within an email'. Worse still - if you go to all the trouble of 'resizing' the image in Thunderbird... it 'looks smaller' on the page... but Thunderbird still sends the full-size file when you press 'send'! It just seems ridiculous.
Anyone who wants to send a full-size file will send it as an attachment, or use WeTransfer, etc., etc. The whole idea of placing an image, or images, within an email is essentially just to convey information or to brighten up the page.
To put this into context, I find myself using Thunderbird as a 'refugee' from Windows LiveMail - which no longer works with gmail. And while Thunderbird is in many ways an excellent email programme, there are two ways in which it still light-years behind now very ancient LiveMail.
One I've just alluded to - its inability to simply incorporate a resized image into the body-text of an email. The second is the choice of type-sizes available. Ancient dinosaur LiveMail offered me anything from 5pt to 72pt and beyond. Thunderbird has 7 choices, and ignores the universal 'point' sizing in favour of tiny, small, normal, big, etc., etc.
Thunderbird has some very sophisticated and carefully considered abilities and qualities. Quite why something a fundamental as inserting a resized image into the text of an email isn't one of them, I've really no idea.