WordPress thumbnails on home page not showing up

I’m hosting my wordpress site on “Amazon Linux AMI 2012.03”. Whenever I upload any custom theme to my website, the thumbnails for the posts fail to show up on my home page.(https://adeln.com/). When I searched the answers community someone advised me to install php-gd, and I did so. It worked fine and thumbnails appeared on my home page. I don’t know know why, after two days again the same problem started. Can anyone please help me to solve this problem. I’m not a professional developer or programmer. So, kindly be detailed. Please tel me what to do and how to do. Thank you :))