Mac with 10.15.7.
When trying to send photos via the yahoo.com web interface via the ‘attach’ commend, the pix is sent as an .heic. Unfortunately some recipients use other operating systems that can not view .heic images.
When trying to send the same file using Mac Mail (the application), IF the image is sent in “original size” it DOES automatically convert it to .jpg. However, if I set the image size to a more reasonable size in the pulldown menu (say, large), it does NOT convert to .jpg but again sends the .heic file.
I do know ways to convert manually, but my partner isn’t into the bits and bytes and just wants emailed images to be viewable by her android and other friends.
What am I missing here, to return her to the golden days of yesteryear when everyone could view emailed photos?
I think, but can't test, that Catalina (10.15) uses HEIC when you send it in a smaller image size, but Big Sur does not, it uses jpeg. Can that Mac be updated to at least Big Sur (11.0) or Monterey (12.0)? That could solve the issue.
Otherwise, you'll need to export the images from Photos (drag and drop works) so they can be jpeg images, and then attach the jpeg images to the Mail message.
Also try dragging and dropping a photo from Photos into a Mail message. See what you get then. I'm not sure what old Catalina did in that case, but it is easy enough to test that.
Another solution may be to stop emailing photos altogether. I don't like to email photos because it forces the recipient to get an email with large attachment(s). If these are Android users, then it is even more likely that they are using a limited bandwidth plan and have slower mobile speeds. Instead, use the iCloud sharing feature in Photos and then send them a simple link. They can then view the gallery of photos in their web browser when they are on Wi-Fi or maybe on their computer instead of on mobile.
As a last resort, you could always just stop storing photos as HEIC in the first place. There is a setting for that in the iPhone Settings app, under Camera settings. Switch back to jpeg. Be a shame to lose the quality and smaller file size though.