Pump it up with your profile pictures – best practise dimension speak

Here are a few basics:

Facebook Picture (Profile or Page)

Profile Picture: 200 pixels wide (100 dpi).
Height can vary substantially. Facebook will convert to a JPG and will shrink images wider than 200 pixels so your page looks best if you save as a JPEG and have an image exactly 200 pixels wide. Since the image spans two colour fields trying to match the background is pointless – in fact, it looks better if you use a third colour as the background, to add some contrast (the blue is #EDEFF4 and the white is standard #FFFFFF).

(Another good idea is to put a 1 pixel border around your photo using colour #D8DFEA. This matches the menu border colour – have a look at the Avid Reader page for an example.)

Facebook Thumbnail
You cannot upload a separate thumbnail image for Facebook, you must zoom into a 110×200 pixel area of your picture that looks good. Think about this when choosing your picture image.

Twitter Picture

Twitter picture: 73×73.
People are able to view a larger image by clicking on your face (or logo)- you may want to do a larger image – still keep dimensions around a variation of 73×73 (i.e. 146×146 or 219×219). Twitter accepts and does not convert a PNG, so use a PNG.

Twitter Background
Because the Twitter website is not fixed in position background image option on Twitter is a design nightmare. Avoid trying to make things line up perfectly on the right side of your profile – rather, put anything important on the left side or have a subtle tiled image. Important: make sure background image colour is the exact match of an HTML colour, and define the rest of the background with that HTML colour. If you do not apply this step the image will look appear as an invisible box.

YouTube Account Image

YouTube account image: 88×88 pixels
Scaling throughout the site at 60×60 (Make sure your image looks good at both) and upload in JPEG format (other formats are just converted to JPEG).

YouTube Channel Background
YouTube backgrounds that are not tiled are centred (and, thus, look ridiculous behind your channel info). If you do a background at all (a solid colour is good), make it something subtle that you can tile nicely. (Note: YouTube is rolling out new channel designs, so stay tuned)

LinkedIn

Profile: 80×80 pixel JPEG works best for the profile image
Company Page: 100×35 pixel JPEG (Other image formats are converted to JPEG automatically even if they are the correct dimensions.)

Flickr

Buddy icon: 48×48 pixel JPEG
Group icon: 48x48 pixel JPEG