The image file isn't deleted immateriality. It's still in the odp
Steps to Reproduce:
1. Launch Impress
2. Insert an image: say https://upload.wikimedia.org/wikipedia/commons/thumb/3/3d/LARGE_elevation.jpg/1280px-LARGE_elevation.jpg
3. Convert the image to a 3d rotation object
4. Save the file -> Check the file size
5. Delete the image - Save again -> Check the file size
6. Close the Presentation
7. Reopen it. Make a edit & Save -> everything back to normal
The file size remains the same, with the image deleted
The file size should shrink after deleting an image
User Profile Reset: No
Build ID: 08441d466dd70c203a519a133aaf1a5997adbbd3
CPU threads: 4; OS: Windows 6.3; UI render: default;
TinderBox: Win-x86@42, Branch:master, Time: 2018-05-06_23:59:40
Locale: nl-NL (nl_NL); Calc: CL
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0
LO 184.108.40.206.alpha1+ (x64) Build ID: 08441d466dd70c203a519a133aaf1a5997adbbd3
CPU threads: 2; OS: Windows 6.1; UI render: default;
TinderBox: Win-x86_64@42, Branch:master, Time: 2018-05-07_01:12:11
Locale: fr-FR (fr_FR); Calc: CL
LO 220.127.116.11 Version ID : 235ab8a-3802056-4a8fed3-2d66ea8-e241b80
under Windows 7 Home.
and upper versions.
So probably inherited from OOo.
Convert to 3D is needed to reproduce.