I just understood there is another topic regarding this issue and I solved my problem according to this instruction)
http://wordpress.org/support/topic/medal-image?replies=9
Copy /plugins/achievements/templates/achievements/achievements/loop-achievements.php and /loop-single-achievement.php into /themes/YOUR_THEME/achievements/.
2) Those two files should now be copied to /themes/YOUR_THEME/achievements/loop-achievements.php and /themes/YOUR_THEME/achievements/loop-single-achievement.php. Open these in a text editor.
3) You might want to add a new column to the table for the picture. Regardless, use the following code in the template to add the picture:
<?php dpa_achievement_image();
Yep — I promise next version will have pictures included, I agree it was an oversight from the original version (I think it was because I ran out of time)!
Would love to have achievement images on the USER profile page (in bbpress), the achievements index (from shortcode), and the leaderboard.
Any kind of update when a new version is coming out that would display the thumbnails? That’s basically all that is needed to make this plugin perfect.
