Is there an "Insert Picture" shortcut for PowerPoint 365 for Mac?

Another Option: Use the macOS Keyboard Shortcut facility to create a keyboard shortcut for the command... Follow the directions in this Apple Support article on how to Create your own shortcuts

Then enter this string exactly as shown below;

Insert->Picture->Picture from File...

There are two ways to solve this.

Add to QAT

1. On the right end of the Quick Access Toolbar, click on the down-pointing arrowhead and choose More Commands.
2. Change Choose commands from to All Commands.
3. Scroll down and select Insert Picture, then click on the right-pointing arrow button to add it to the right-hand list. OK out.

Add to Ribbon

1. Choose PowerPoint>Preferences>Ribbon & Toolbar.
2. In the right-hand Main Tabs list, select a tab to which you want to add the command.
3. Click the Plus sign at the bottom of the right-hand list and choose New Group.
4. Change Choose commands from to All Commands.
5. Scroll down and select Insert Picture, then click on the right-pointing arrow button to add it to the right-hand list. OK out.