Welcome to LibreOffice and this forum.
In the table that you want to insert pictures - make a field with a field type of Image [LONGVARBINARY].
In your form or report, insert the Image field and size it however you desire.
You enter pictures into this field by right clicking on the form field and selecting Insert Image from . . .
It then opens the file file browser for you to find and select the desired image file.
Once the field is populated you get an additional option to Remove Image, if desired.
Keep in mind that large image files will make a large database table that may slow things down a bit. Try to keep the picture resolution as low as possible (smaller file sizes).