Missing images in Keynote, It shows squares with x's through some of them.

Hello PabloA.S,

We can understand your concerns with your presentations not working as expected. You mentioned that you recently upgraded the macOS. Thanks for letting us know about this. Were the photos included in your presentation items you have saved in the Photos app on your Mac? If so, you may want to check if the issue is related to your Photo library.

This article might be helpful if you need to designate a systems Photo library on your device:

Designate a System Photo Library in Photos - Apple Support

How to designate a System Photo Library

If you designate a new library as the System Photo Library and then turn on iCloud Photos, the photos and videos in the new library will merge with those already in your iCloud Photos. Additionally, all photos and videos from iCloud will redownload to the device.

1. Quit the Photos app.

2. Hold down the Option key and open Photos. One of the photo libraries is already designated as 3. YourLibraryName (System Photo Library). 

3. Choose the library you want to designate as the System Photo Library.

4. After Photos opens the library, choose Photos > Preferences from the menu bar.

5. Click the General tab.

6. Click the Use as System Photo Library button.

If Use as System Photo Library is grayed out, the current library is already set as your System Photo Library.

If you open a different library in the Photos app, and you haven't designated it as the System Photo Library, other applications will use photos from the original System Photo Library. Hold down the Option key when you open Photos to see which library is set as the System Photo Library.

Hope this helps!

Best regards.

This also has happened to me when updating. Any images that were pasted into my keynotes before the update appear completely blank.