Post Image not displaying in category view

add_theme_support( 'post-thumbnails' ); only adds the ability to use Featured Images. To have them appear in your site, you’ll need to add code to at least one of your template files, your active theme’s functions.php file, or a custom plugin.

// in The Loop
if( has_post_thumbnail() ) {
    the_post_thumbnail();
}
// etc.

References

• has_post_thumbnail()
• the_post_thumbnail()
• Post Thumbnails (now called Featured Images)
• Templates
• Writing a Plugin