They can't handle Macs' inline images so help them out
This comes up a lot. When you email a photo, Apple shows it to you and to them right in the message: it isn't just a file attachment, the actual image is visible. If your recipient is on a Mac, they see the photo and can detach it as normal. If they're on a PC, they might be able to do that but often they can't. Often all they get is the image in the message without any way to detach it, drag it or do anything really apart from complain to us.
This does not work
Perhaps you've tried this already. When you're composing an email to someone and you drag in an image, it displays at full size. You can right click on it and say you want it to be displayed as an icon instead. Apple Mail then does this and, nicely, does it for all the images you've dragged in so far.