The logo is an attachment. Anything in an email message that is not text must be sent as an attachment in order to be compliant with email protocols. But many modern email programs will automatically display the contents of some attachments. Apple Mail will display JPEGs and single-page PDFs. Presumably your correspondents are using an application that displays JPEGs but not PDFs.
Some email programs expect attachments to come at the end of the message, not intermixed with the other contents of the message. Such an app may treat everything after the first attachment as attachments. That might cause the different behavior when additional attachments are included before the attachment in your signature.