the background image problem on impress

there is a ppt file, which is saved on libreoffice 5.4.x as ms powerpoint 97-2003.
I used to change the background image via “set background image” command in context menu and “apply to all slide”.

Libreoffice 6.x doesn’t have the command in context menu. So to do the same thing, I have to remove all background images on all slides and set the background image on the master slide.

I know, on the purpose, using master slide is right. But I didn’t use master slide before and I have many presentation files. Is there any good solution to fix this?

example file :

In LO 6.x : in the context menu choose Properties then the background tab to change the background of the current slide.
To change the background for all slides, the correct way is to edit the master slide. It is very easy to do from de sidebar.

thank you, but as I already wrote on my question, I know what the proper way is. but I have many files which have the same problem. So my question was “how to fix them of many files easily.”

  • Save your ppt file as odp file and rename as zip file
  • Open the zip file on a zip editor and move to pictures
  • Find some jpg images and edit the right one (background) by an image processing application
  • Save the altered image and the zip file as well (you are asked for that)

After renaming into an odp file the presentation can be run again.

In JBF’s solution the original jpg is still there in the pictures folder, but the proceeding is much easier. LibreOffice 5/6 can show the alteration but not ApacheOpenOffice. AOO still shows the original background.