regex to replace links that ends with .jpg,.gif or .png

Hi, not sure if this is the right place but im trying to get a regex function that replaces image links .
i.e i want to replace
http://forums.asp.net/Themes/FAN/style/i/logo.png
with
http://imagetranscoder.site/?img=http://forums.asp.net/Themes/FAN/style/i/logo.png

this is required for me to use some transcoder to resize images to small screen size ...