EDIT: the comment about "most compatible" was based upon information from the article, rather than access to an iPhone. I have since looked it up, and JPEG is mentioned underneat the option.
I am saying this because people need to have a degree of understanding of technology in order to have some control over it, even though I recognize that some people will construe such statements as being elitist. That depth of knowledge does not even have to be particularly deep. In this case, understanding that photos may be represented in different ways by the computer and that you have to ensure that the recipient can accept that representation is important. After all, this is not the only case where they will run into this issue. It is a big part of the reason why businesses settle upon some form of standard for the exchange of data, may that be through common business practices or a standardization body.