If you want to find out more about the symbology of Guernica, you can check out the Wikipedia entry on Guernica(C2 level), or read Dominic Witek's short and sharp interpretation of the art piece in Artsper Magazine, which is entitled Artwork Analysis, Guernica by Picasso. This text can also be accessible to C1 students and above.
As for a visual alternative, you can watch Picasso's Guernica: Great Art Explained, a thorough 13:40 video with subtitles for C1 students and above, which delves into the iconographic structure of the painting and compares Guernica to other works or art: