There is a bug in the HTML portion of the email created on an iphone in IOS14. This is something that iphone users cannot control and must be fixed in the iphone mail tool. It may have also been a bug in earlier versions of IOS. The problem is this: The image is embedded in the HTML portion, but there is a keyword missing in that part of the email. The missing keyword is: <!DOCTYPE html>
If you look at the "raw source" on the Apple mail tool on Macs (Mail->View->Message->Raw Source) when viewing the email, you can see the error. See below for an example of the beginning of that part of the email. The image is embedded there, but if <!DOCTYPE html> is missing, then the mailtool will not display that version of the email. Instead, it displays the first part of the email with plain text and no images at all.
I already filed a bug report a few minutes ago.
There is a workaround once a user receives the email on their Mac. They can select the email in mailtool, then select File->Save As and choose Format "Raw Message Source". This creates a file with .eml at the end of the name on your desktop or wherever you select. Then you can edit that .eml file with TextEdit (don't just double-click to open it...instead, right-click/two-finger select the file and choose Open With->Other and scroll down the list of applications until you find TextEdit.app, then choose that). In TextEdit, insert <!DOCTYPE html> in the correct place and save the file. Then go back to mail tool and choose File->Import Mailboxes, choose Import data from Apple Mail, then Continue, then select the file you changed. This creates a new mailbox with a name like Import or Import-2. Select the mail item with in it and you can see it with the change, and the image should then show up. You can then move the email into your inbox or forward it or whatever you want to do with it.
I should point out that there is another problem. This worked find when the embedded image was "jpeg" on Mac OS X Mojave, but when I did the very same thing with an IOS email that embedded a "heic" image (Apple's own image format I think?), the image still does not display when viewing in the mac OS X mailtool. It seems heic might not be supported on Mojave? But you will see that other images and fancy html will work whereas it wouldn't have before.
Sample of *corrected* syntax of the content of the email, with <!DOCTYPE html> inserted after the Content-Type statement and before the <html> tag:
--Apple-Mail-F085916F-874E-4FE6-82C8-2EAB3371F4A4
Content-Type: text/html;
charset=utf-8
Content-Transfer-Encoding: quoted-printable
<!DOCTYPE html>
<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charset=3D=
utf-8"></head><body dir=3D"auto"><div dir=3D"ltr">
<div><br></div>
<div>Fry 1 pound diced chicken in peanut oil.</div>
(etc.)