Complex sentence: She suspects Shin-ji tampered with her cosmetics so she could be the lead, and uploads unflattering pictures of Shin-ji to the internet decreasing her popularity and insuring that Ah-rang would be able to hold on to her spot as lead.
Split into simpler sentences: She suspects Shin-ji tampered with her cosmetics so she could be the lead. In turn A-rang posted pictures of Shin-ji on the internet decreasing her popularity and insuring that Ah-rang held on to her spot as lead.