EDITING: If you insert an image into a protected table over a non-protected cell, it cannot be manipulated or deleted.
Make the unprotected cell the active cell. Use Insert > Image and make sure, that the anchor is set to cell, not to page. After the image is inserted, it cannot be accessed. This happens even if the cell is so large, that the image would be totally inside the cell. If you try to insert a shape, e.g. a circle, into the unprotected cell, you will notice, that it is not possible. Cell anchored objects are neither accessible, if you first insert them, and protect the sheet afterwards. In case it is intended, that an object which is anchored to an unprotected cell is not accessible, then it should be forbidden to insert an image. But I think it would be a more user friendly, if objects, that are anchored to an unprotected cell can be edited. The current behavior is inherited from OOo. A similar bug report exists as https://bz.apache.org/ooo/show_bug.cgi?id=101866
If the whole table and every cell is protected you could also insert every image or drawing object, which had been copied to the clipboard, by CTRL+V. There is no possibility to mark this object any more. You have to change to unprotected mode and then delete the inserted object.