One of the things that designers need to keep up with is all the different branding guidelines and specs from all the different networks and services like Facebook, Twitter, Google+ and Flickr (just to mention a few).
I have developed several PSD freebies over the years, but a site I’ve found useful (mainly to take out the specific sizes I need the images to be in) is Social Media Image Maker by Autre Planete. They have this smart site that lets you resize and slightly retouch your images.
These are the services they set up for you: Facebook, Twitter, Youtube, Google+, Flickr, Vimeo, Pinterest, Skype, Tumblr, LinkedIn, Gravatar, Xing, Viadeo, Slideshares, Foursquare, About.me
Give them a try!