If you really want to test it properly, you need everyone to send exactly the same image. Otherwise, you’ll never know for certain whether it is a difference in the sender, or the image you get.
Also, email clients often reduce the size of images they send. When creating an email In Mail on iOS you can tap the CC/BCC line to expand it. If there is an image in the email you will see an Image button to the right, and be able to pick from various sizes.
Since the whether something is displayed in the body has a lot to do with the size of the attachment (my main theory) you’d need to make sure everyone is sending the image in the actual size.
PDF’s are different. They tend to show single pages in the body and mulitple page documents as attachments.
To re-emphasize, the files are not being sent differently, they are just being displayed differently. This is something most modern email clients do, especially with pictures which most people like to be able to easily see in their emails rather than having to save and open them. There should be a way, either through the message menu or by right-clicking to do anything with the images in the body that you could do with the little attachment icons.