Hi Tony.  

I think that the main reason ppl are not doing it is because many don’t know how and it takes too much time probably without much added benefit. 
The one down side I can think of load time, 

I think at the end of the day, it is the content of the image and not the size that attracts people.  But, you should not ignore the idea that appearing too rigid could be a turn off for some people although I think that would result from “picture perfect framing” and “uniformed images” etc and not the image it self.

Also, remember that there was a time when the way images appeared on a page was different than the current layout.

I myself am in the process of updating one of my new pages where I uploaded uniform tabs – some of the feedback I got was that each tab is completely lost since they all seem to run together because they all have an identical look. To address this, I intend to create frames for each that are identical but fill each with a unique image that represents the content. The frame should pull them together without losing the identity of each.

You can see what I am talking about  here for the next day or two:  http://facebook.com/savvysocialmediamanager

and I am inserting an image just in case I have changed it before you take a look:

HTH and all the best.Gina

I work for a large company and I adjust my images to fit timeline, the feed, and highlighted timeline items. a good size for me is about 400w x 300h when including text with an image because it will show up nice in your timeline and large in the news feed.

@Andrea  - of course I know this and realize I used the cover I made for my Google+ page instead……..(too many spaces and rules to keep track off . Now that I am trying to upload the correct image FB is not letting me do it for some reason  urgh!!!! I’l try again later.

Thanks for alerting me.