[Theme: Sahifa] Thumbnails Original Size

Hello, when I post a new article instead of my thumbnail to be resized it’s the original size of image I’ve put in article, this looks very bad, I tried to regenerate my thumbnails but all did is to broke my all articles thumbnails size.
I checked to see if when I upload a new image is creating different sizes of image I’ve uploaded and it doesn’t, also sahifa theme hasn’t any “regenerate” button.
What can I do to repair my website thumbnail size?

Thank you, hope for a fast solution