Your company email signature incl. logo and other images.
When emails sent from Outlook on Windows (or Xink fully server-side with embedded images) and replied from Mail on iOS or Mail on OS X, the images are removed because images are embedded in the email and Apple email clients does not support embedded images which was developed by Microsoft and launched in Outlook 2007 version.
This is standard behavior for Mail App on all iOS devices (iPhones and iPads) and Apple Mail on Mac OSX.
When you reply or forward an email, sometimes email signature images are missing and replaces with image tags at the recipient end:
Move to Microsoft email clients; Outlook for iOS and Outlook on OS X which support embedded images where no Red X will show before the images are downloaded.
Alternatively, switch to linked images and continue to use Apple email clients; Mail app for iOS and Apple Mail for OS X.