In High Sierra, jpegs attached to mail are received as icons

Thanks, dialabrain. The recipient in this instance was myself. I sent the email with the attachment to myself in order to check that the image would be shown. So the mail client that received the mail was also Apple Mail. (In High Sierra).

That looks very interesting, Sparkleberry; thank you very much. But where do I find these settings? I just dragged the jpeg into my email to attach it, and I saw no menu of settings.

As far as the recipient’s mail client is concerned, the recipient was myself (in order to test how the received mail would look) and so the receiving mail client was Apple Mail (High Sierra) just like the app in which the email was created and the image attached in the first place.

Sparkleberry and dialabrain, I should have explained what I was doing more fully. I was creating a template for seasonal greeting cards (following advice on a website I had found) by creating a Templates mailbox, building the card as an email, saving it, dragging it from Drafts to Templates and — to send it — using Messages/Send again.

It turns out that dragging it is what caused my problem. If, instead, one uses the 'Move' tool in Mail's tool-bar, the attached image(s) are retained in full visible form.

Also, the Templates folder must be located on the Mac; if it is located in one's email account, using the 'Move' tool to put the email there makes the email disappear (though dragging it there does not do that, and has the advantage that the Templates folder and its contents become accessible in one's other devices linked by iCloud, though then images get replaced by icons).

Thanks for advice and comments.