I have just downloaded the compression add-on that was shown in the list of available add-ons for Thunderbird. I tried to compress a large photo file in a test email and the add-on did not recognise the file type. It was a JPG file of 4.38 MB. I have problems sending files of this size, which are not always welcome, and when I do send one it then has problems being transferred to my Sent file. I used to use Picasa which had an option to send photos directly (i.e. not attached to an email) in different sizes, but Picasa is now no longer supported and Google Photos does not have this useful feature.