New smileys are not added

It is not certain how to add smileys. After an attempt to add one or more smileys, a message, that the addition is successful, is given, but actually nothing happens.
Steps:
1. I upload the smileys to root/images/smilies/, for example icon_duh.gif and icon_yahoo.gif;
2. I go to the ACP;
3. I click on the tab POSTING;
4. I click on Smilies;
5. I click on Add multiple smilies;
6. The two new smileys are listed in the 3rd and 2nd row from bottom;
7. At each smiley I do:
7.1 Under Smiley code I type ":duh:" or ":yahoo:";
7.2 Under Add I click the checkbox. It gets checked;
8. I click on Submit;
9. I get the message: "Information The smiley has been added successfully."
10. I try to make a post with :duh: and :yahoo:.
11. The smileys are not listed in the tables.
12. I type :duh: and :yahoo: and click on Preview.
They are not interpreted, but they are displayed just exactly as I typed;
13. I click on Submit.
In the message they are displayed exactly as I typed.