htaccess changes doesnt respnd the way as it should to clean URL

htaccess changes doesnt respnd the way as it should to clean URL

I have bunch of URLs
http://www.mydomain.com/aboutUs.php
http://www.mydomain.com/contactUs.php
and so on....
The Rewrite Rule I wrote in my .htaccess file is
RewriteEngine on
RewriteRule ^([a-zA-Z0-9]+)$ contactUs.php
RewriteRule ^([a-zA-Z0-9]+)$ aboutUs.php
and so on...
Now when I use the clean URL, let say for example,
http://www.mydomain.com/aboutUs
It points to homepage always...i.e. http://www.mydomain.com/, BUT the URL
remains http://www.mydomain.com/aboutUs
And similar behaviour for other URLs as well.
Also the images in the banner doesnt appear while using clean URL... As I
have used a src attribute for images as <?php echo
HTTP_PATH;?>images/header-logo.png
Where, HTTP_PATH = "http://" . $_SERVER["HTTP_HOST"])
Could someone guide me whats wrong with the .htaccess or is it anything else?
UPDATE as per @HAKRE Comment
I need the URL to look like http://www.mydomain.com/aboutUs and it should
point to http://www.mydomain.com/aboutUs.php