Seems to me you use a WordPress theme which uses some kind of php library to resize images. You should really use WordPress default add_image_size() and wp_get_attachment_image but if you were the one who created the theme you would already know that.
There are 3 things you can do:
- find a new theme that is not that outdated
- dive into
mytheme/thumb.phpand find out whats going on and where it fails - update the theme wherever it uses thumbnails and/or different image sizes with the above default WP functions