I see the problem: when the image can be resized by clicking, the embedded link is inactive. For the recipient, the link becomes active if you zoom out until the image fits the window (View/Zoom/Zoom Out (Ctrl+- or Ctrl+scroll mousewheel)). Uncheck Zoom Text Only.

Most links are embedded in small images or thumbnails, so this is probably fairly rare. I don't know what you should tell the sender, except maybe to embed the link in a string of text below the image, like a caption.

tx. i'm the sender, and on occasion would like to set an image's link attribute when the image is larger than a thumbnail. for example, this happens when i want to snip an image of a front-page headline, picture, & article summary from an online newspaper, and set the image's link attribute to point to the full article.

and, yes, if i have to tell the recipient to zoom out or inflate the window in order to be able to click the image and get to the linked page, it's kinda self-defeating... so i'd give up on that feature & instead follow your suggestion of eg including linked text below the image.

(where is the "zoom text only" setting?)

i would have thought that, when i compose the email, there's some setting (accessible even if i have to change to email's markup) that tells the receiving email client not to disable the image's link, eg by doing what the thunderbird client does. if there's no such setting, then that's puzzling... but it is what it is.

I think it's better to not embed links in the images unless they are very small. You can't know what size or resolution the recipient's screen is set to, e.g. it could be a phone or a TV, and different mail apps might have different reactions to clicking oversized embedded images.

There are add-ons to resize images, such as Auto Resize Image, if you prefer to do this in TB.

The Zoom Text Only option is under View/Zoom.