From the first tests it seems to me that LibreOffice on KDE gives a lot of strange problems and malfunctions when trying to insert an image. If I didn't make some mess...

You didn't say whether you were using writer, draw, etc etc etc.
But in Writer I went Insert/Image, I selected it, and poof it was inserted into the document. I believed I would have had to make an image box first, but I did not.
I'm in Budgie. Describe more, you don't have any specific information (two cents).

    brent Both with writers and drow. Insert – Image-> and I selcet a jpg of a screenshot of 81 kb.

    It seems as if the image is there, but you can't see it and everything freezes a bit.
    I checked on the option Tools : Options : LibreOffice Writer : View : Display and "Images and objects" is checked.
    I don't understand where I'm wrong.
    I don't know if this means anything useful:

     ~ $ soffice
qt.gui.icc: fromIccProfile: failed size sanity 2
qt.gui.icc: fromIccProfile: failed size sanity 2
QTimeLine::start: already running
kf.kio.core: An error occurred during write. The worker terminates now.
qt.gui.icc: fromIccProfile: failed size sanity 2
qt.gui.icc: fromIccProfile: failed size sanity 2
kf.kio.core: An error occurred during write. The worker terminates now.
qt.gui.icc: fromIccProfile: failed size sanity 2
qt.gui.icc: fromIccProfile: failed size sanity 2
qt.gui.icc: fromIccProfile: failed size sanity 2
qt.gui.icc: fromIccProfile: failed size sanity 2
kf.kio.core: An error occurred during write. The worker terminates now.
qt.gui.icc: fromIccProfile: failed size sanity 2
qt.gui.icc: fromIccProfile: failed size sanity 2
kf.kio.core: An error occurred during write. The worker terminates now.
qt.gui.icc: fromIccProfile: failed size sanity 2
qt.gui.icc: fromIccProfile: failed size sanity 2
kf.kio.core: An error occurred during write. The worker terminates now.
qt.gui.icc: fromIccProfile: failed size sanity 2

      I'm in Budgie. You are in Plasma.
      I can insert an 81kb image, see:

      but you cannot insert.
      This is my output, normal:

      ~ $ soffice

(soffice:10419): Gtk-CRITICAL **: 12:56:08.215: gtk_widget_get_toplevel: assertion 'GTK_IS_WIDGET (widget)' failed

      your output is not normal, to me, and I wonder if a DEV or similar could explain all that sanity stuff of yours:
      qt.gui.icc: fromIccProfile: failed size sanity 2.
      When I search engine "qt.gui.icc: fromIccProfile: failed size sanity 2" I am seeing a lot of graphic apps will show that error.
      this is from January:
      "Looks like it's a problem in the image file used for a profile icon -- see https://stackoverflow.com/questions/65463848/pyqt5-fromiccprofile-failed-minimal-tag-size-sanity-error?noredirect=1"
      seeing a lot of ICC profile stuff too.
      I really really hope someone better informed than me can tell you why you 1) can't insert an image in Libre Writer and 2) and if those errors are related. At the end of my brain here, I am sorry.
      If I see anything further I will post more.

      https://duckduckgo.com/l/?uddg=https%3A%2F%2Fdocumentation.libreoffice.org%2Fassets%2FUploads%2FDocumentation%2Fen%2FWG4.2%2FPDF%2FWG4208-WorkingWithImages.pdf&notrut=duckduck_in

      according to the LibreOffice document here called "Working With Images" they say "Images can be added to a document in several ways: by inserting an image file, directly from a graphics program or a scanner, from a file stored on your computer, or by copying and pasting from a source being viewed on your computer."

      then they list how to do these 4 methods...their #1 way does not work for you. What about #2, #3, and #4?

      My guess if none of them work let Solus know...if one of them works you have a workaround but the main function will remain broken. I agree, it should not be a battle. Something is askew for sure.

        Budgie no errors or issues with images.

        brent Thanks for your generous help, the images were 8-bit, with ICC GIMP built-in sRGB. Of the proposed systems on the guide of your link one works (!), the one named "Inserting an image from the clipboard" : to open the image directly with the File-> Open, in this way the image opens in draw and you can copy past the image like you want. So it seems like a problem related to this PyQt5..., but at least there is a trick to avoid it.
        Thanks!