Re-edit an embedded object in an Outlook message

I created an Outlook message with several graphics in it. I created each graphic by embedding a PowerPoint slide, and then editing its content and size.  I then sent the message, and the recipients could see the graphics clearly.  So far so good.  (I chose to embed slides because Outlook's graphics editing features seem considerably less powerful than PowerPoint's)

Now I want to produce a revised version of the message, to send to other people.  I've told Outlook I want to Forward. Editing works great on the message text. But I can't edit the embedded PowerPoint slides because they have all turned into pictures.

How can I get put a graphic into Outlook that I'll be able to continue editing (with a PowerPoint-grade editor) over multiple Send's?