I believe your problem lies in your filename regular expression:
The underscores are incorrect.
You need to allow all letters, all numbers and underscores. Fortunately there is a shorthand for [A-Za-z0-9_] which is \w:
RewriteRule ^wallpapers/(\\w+)\\.(jpg|gif)$ /img_content/extra/$1.$2 [L]
If that shorthand doesn't work in APACHE (it normally works in PHP and Perl), try the following:
RewriteRule ^wallpapers/([A-Za-z0-9_]+)\\.(jpg|gif)$ /img_content/extra/$1.$2 [L]