I find myself looking for this information all the time, so here’s a quick cheat sheet.
On the current date, 5 April 2013 here are the pixel dimensions for the header images on Facebook profiles, Google+ pages, and the header image for WordPress (Weaver theme).
This image is a snippet from a full and super-helpful infographic by Jon Loomer:
The new image size for Google+ is 480 x 270
And for WordPress Weaver header images, the dimensions are: 940 x 198
Ooops, almost forgot Twitter profile banners. They are ideally 1252 x 626 pixels and cannot be larger than 5MB.
I find that an easy way to be able to change out my header images myself (without a graphic designer’s help) is to setup custom cropping sizes in Picasa and label them accordingly (eg. put in the dimensions for the Facebook cover image and name the custom size “facebook cover”)
That way you have an easy way to crop any photos you’ve imported from your phone or camera. You can also use Picasa to create a collage of images and then have it create the collage to match the dimensions of the image you are trying to create. You can even add text, or a watermark to your finished image easily.
Here’s a quick Tutorial on creating profile images in Picasa:
Questions? please ask below in the comments section.