Open (or wait a bit) 2.23 (or older) and press "Yes" when asked about the update to open the update page with all updating information.

v.2.24

• Fixed some international domains validation.
• Fixed forum prefix tag change to default (if was changed before) when setting prefix id and icon id to empty.
• Improved. When removing banned host clean up the empty [color] tags.
• Added cover (featured image) support for some Wordpress blogs ("resim" input name for image url).
• Fixed some various small posting problems.
• Improved. First update version notice is now shown after program is fully loaded.
• Added new setting in "CONFIG" -> "Other" -> "Default left tab on start-up". Which tab is by default selected on start-up.
• Added ability to change prefix for multiple groups / sections at once.
• Added option "Visualize images without [img] tag" for message preview window.
• Added 3 new build-in forum.
• Other small improvements.

For previous versions changelog: click here

How this featured image acctually work with thema poster, i need explanation?

Read about cover here: http://poster.freddy.lt/faq.php?expand=faq103

As it says in the FAQ if that setting is enabled you probably can just forget this and don't worry at all. I actually seen only one Wordpress site which has these "Cover" things.

This settings is in config/general posting so enable it would effect for all forums/blogs?

How can i enable it only for wordpress sites?

This is wrong place to ask. There is help forum: viewforum.php?f=4

Those are global settings, I'm not sure why would you only want them for Wordpress, they are fine to use for all websites and I never heard anyone who would want only for Wordpress.