but how do I "associate" jpg files for Thunderbird to send them ..

You don't. Thunderbird doesn't care what file extension is used when sending attachments. The file extension (and hence the file type) is important when opening received attachments with a certain application.

Did you install a specific add-on for Thunderbird in your Win7 Thunderbird installation?

What's more likely to me is that you used some sort of 3rd party software for this, but I'm just guessing.

Alternatively you can use the FileLink feature for sending large attachments like photos. https://support.mozilla.org/en-US/kb/filelink-large-attachments The default provider box.com is still broken in Thunderbird 38 (will be fixed in v45), but Dropbox works well. https://addons.mozilla.org/en-US/thunderbird/addon/dropbox-for-filelink

See this article for more information on installing extension in Thunderbird. http://xenos-email-notes.simplesite.com/416814616

no I don't think I ever installed any further software. It goes through the process of preparing to send but fails at that last error. Is it Thunderbird that is producing the error or is it win10? I have got the exact wording as follows: There is no email program associated to perform the requested action. Please install an email program or, if one is already installed, create a file association in the Default Programs control panel. Just to confirm, Thunderbird is the default email program in win10 ..

Is it Thunderbird that is producing the error or is it win10?

I don't think so. Not using Win10 here and I don't really understand what Windows function you're calling and what file association Windows wants.

In Win10 things have changed wrt how the default program is assigned. So even if you think Thunderbird is your default email program, that may not be the case. You may ask about this in a Windows forum.