The method of inserting an image - of any size - into an email would work as follows:
1. Place the cursor where you would like the image to appear on the page.
2. Click 'insert'/'single image', from the top menu bar.
3. A folder from one of your 'libraries' will appear - navigate to the folder with the image you want to use.
4. Select an image - click 'open'.
5. A moderately sized image will appear where your cursor was positioned. (NB. The image has handles and can be 'resized' if required. A variety of box rules or frames can also be added to the image).
6. If this all sound insanely 'simple and straightforward', it's the system for inserting images into an email that I've used for the past ten years or more, but now had to abandon because I'm using Thunderbird.
In 2022 I find it hard to believe that I'm putting this up as 'an idea'; but I find Thunderbird's 'system' for inserting an image into an email to be almost unbelieveable. Even if you go to all the trouble of manually 'resizing' the picture - it still tries to send the original file! - which nowadays is often around 7MB - for 1 picture.
Anyway - if anyone would like to bring the utter simplicity of the method I outlined at the start to Thunderbird's interface... - it does exist, and I would guess someone with the appropriate skillset could just lift the code and make it part of Thunderbird.