Image thumbnails in posts invisible

Hello guys.

Well, I have downloaded a theme, and I have redefined it according to the documentation. Now the problem is whenever I am posting something, the image thumbnail is not showing. But, when I open that particular post, the image is visible there. Does any of you know what the problem could be? This is happening with all the posts and different images also.

I have tried uploading the images at the the wp-content/uploads folder even with fail.

I am sharing two pictures for you to understand.

Please if anyone knows the solution, help

NOTE: I have tried using some thumbnail plugins, failed
and the theme I have downloaded has a thumb.php file, I fear modifying that as i have no clue about php.

Post’s page : https://imageshack.us/photo/my-images/189/62269707.png

Home page : https://imageshack.us/photo/my-images/51/54580436.png