Tried right-click on the image?

In tools /options / Privacy / Mail content -section You can allow remote content..... If you are willing to take the risk.

You should also have this on top of your mail.

Gnospen: perhaps I did not explain the problem clearly. The email in question was created in Thunderbird with some text followed by an embedded image (Insert-Image). In the process of inserting the image, I added a mailto: link to the image intending that the recipient could click on the image and open a new mail message with a preset To: address and subject line. (the mailto: text includes ?subject=). I then sent this email to my Thunderbird account as well as my gmail, aol, and yahoo accounts. In the latter three accounts, clicking on the image produced the desired result. In Thunderbird, the text and image are displayed when the email is opened. The cursor changes to a magnifying glass when over the image and clicking on the image only enlarges or reduces the image size. In Thunderbird, the mailto: link, which is displayed at the bottom of the email window when the cursor is over the image, cannot be followed.

Zenos: thanks for your suggestion. Right-clicking on the image produces a dropdown list but does not follow the mailto: link, which would open a new email addressed as specified in the mailto: link. Why am I seeing the magnifying glass icon as cursor when I hover over the image? I don't remember that happening before.

Performed a test: Open Write Type Subject 'Testing mailto'

Insert > Image click on 'choose' select image file and click on 'Open' click on 'Dimensions' tab choose size. I selected 'Custom size' and 'constrain' set width to 200 pixels as it wider than height. Click on 'Link' tab enter: mailto:mail at domain dot co dot uk?subject=go test this (where 'at' = @ and 'dot' is a full stop/period and there are no spaces until the actual subject) click on OK

Save as 'Draft' Click on 'Drafts' folder select email so it shows in Message Pane hover over links looks ok click on image and it opens a new Write window.

Then I sent to myself - other email address. Message received. click on image and a new Write window opens with TO field completed and Subject completed.

Toad-Hall - thanks for doing that test. I have found the answer to my problem, which has nothing to do with the way the email was created but rather is a function of how I was viewing it. The image in the email is rather large (1275 pixels wide by 1650 pixels high). I always have several windows open (Win7) in varying sizes. The Thunderbird window in which I was viewing the problem email was not as wide as the image. When I mouse over the image, the cursor turns into a magnifying class and I am unable to follow the mailto: link. However, if I widen the window beyond the width of the image (i.e. the whole width of the image is visible in the Thunderbird window), then the cursor becomes a pointing finger when I mouse over the image and a left-click follows the mailto: link and opens a new email with the appropriate address and subject.

I was unaware of this magnifying function so I did not recognize what was happening. I'm not sure if this is a good way to do things or not. Do you think this is a bug or simply a hidden feature?

image zoom is nothing new, at least to Firefox users, which you appear to be.

Image scaling is, or should be the root cause here. Where the image does not fit, it is scaled to fit this scaling comes with a zoom to bring the image back to full size. An example of it in action in Firefox is here http://cache.desktopnexus.com/wallpapers/1592/1592605-2048x1367-7.jpg clicking the image reverts it to it's native size from scaled to fit. Using this image in a Thunderbird mail will in most cases force scaling as it is desktop size or more for most desktops. This functionality was introduced as a feature of Thunderbird 31. in Bug 534083

Whilst it might be a pain in this instance, it means that those huge happy snap images granny mails will be scaled so you can see them when they arrive instead of only the smallest part of the top left corner.