How to put a Picture Full Size on the home page.

[ant3840 0]

I'm sorry for my english, i don't know how to make the title for this make sense. I want to put a picture in the Home page for all the post, like this http://www.adventurekayaking.org/?category_name=kayaks, i put the picture but it looks CROP?. BUT i Don't want to put the pic in the BLOG POST, in all my post i'm going to put video, and some text, but i only want to put the Picture in the Home page like a presentation for the post. Other thing is the Picture Is ON TOP of the Title. ?????? That don't look good, hhahha. You can see in my site what i'm try to say http://www.aprendermagiagratis.com Again sorry for my english, and thx in advance.