Before you’re able to add an image to the background of email messages,enable HTML formatting.
A background image can only be added to individual email messages.
Reapply these controls for each email you want to have a background picture.

Lifewire / Derek Abella
pop fire up Microsoft Outlook desktop app.
Go to theHometab and selectNew Email.
In the new message window, place the cursor in the message body.
In theThemesgroup, select thePage Colordropdown arrow.
In theFill Effectsdialog box, go to thePicturetab.
In theFill Effectsdialog box, selectOK.
If you don’t like the background image, remove the image from the new message.
Go to theOptionstab, select thePage Colordropdown arrow, and chooseNo Color.
Or, prevent thebackground imagefrom scrolling.
Click in the body of the email.
ChooseOptions>Background Picture.
grab the image you want to use as the background picture, then clickOpen.