Blogosphere is an amazing world, with each blog speaking its own mind, having its own personality. Your profile picture and your profile name is the window to the reader to your little world. Do you give enough thought before choosing a picture ? Do you know it can have tremendous effect on the reader's decision or otherwise to check out your blog or not?
Various people choose many types of pics - self, other's, cartoons, animals, things, slogan etc.

CELEBRITY STUFF : Either you love the celebrity too much (ideally you should find a better way to express it!). But seriously speaking, it shows non seriousness towards blog and also lack of confidence in one's own looks or personality.

NATURE AND OBJECTS : Same goes for things of natural beauty, like sky, clouds, river, fire. Here, for example,boundless sky is an expression of one's broad mindset etc. Objects like earthen lamp or light signifies hope in the time of despair or being a self made person.
PICTURES OF SELF/HUMAN FACE. This is one sphere which needs maximum elaboration and its quite interesting.
A passport kind of a pic shows someone very serious, especially serious about one's image and one's blog and wants a proper representation of his identity on his blog.
A close up of the face of the blogger with light, smiling and pleasant expression shows the person is social, with straight forward approach towards the blog and has nothing to hide about him/herself.
A Side cut of a face, musing, thinking, shows the person either wants to partly hide his identity and secondly wants to highlight the thinking, ponderous side of his personality. If thoughtfully chosen by the blogger, then such characteristics like deep observations and thoughts would be visible in his/her blog too. (This one is my personal favourite!)
PICTURES WITH FACE HIDDEN (PARTIALLY)
YOU DON'T KNOW ME :Hiding the face completely means the person does not want to reveal her identity at all. But relates to her blog hence puts her touch to the pic by darkening her pic, avoiding the exposure of face but pics of body parts like hands, feet may used. It also adds an element of mystery and the pic remain very individualistic.

EYES : This is most commonly seen, one eye in close up. (That makes me laugh always, it looks like retina scan to me!) Its weird and that is what the blogger wants to show
NUDITY : Display of obscene gestures, nudity or showing off one's intimate body parts, even partially, is all attention seeking behaviour or else it can mean that the person is giving a window the the content of the blog. ( can see the glee in your eyes, u guys!!)
MESSAGE FREAKS : Some people like putting message/thought on things like a T-shirt as profile pic. Again shows need for anonymity and telling the reader his/her attitude about a thing. Moreover its quite imaginative also.
GIRL IN THE CLOAK : sometimes, in order to avoid the attention of non-serious male readers/bloggers (who are mouse-clicking happy to see a pretty face!), girls choose profile pics which are non-descript.
ATTITUDE - HAVE IT FLAUNT IT - some profile pics show certain body language showing highhandedness, arrogance, superiority or "couldn't care less" attitude, say a pic of lower limbs of a man sitting with feet up on a working table. Its OK if you have attitude, but then that should be the shade of your blog too.
The only point I want to make here is that its alright to have a profile picture whatsoever you choose, after all you are the best judge and its your blog, but having a profile picture, and for that matter a profile name too, congruent to your blog, the flavour, the feel of your blog is important. After all,it can affect your blog traffic, when the pic is too bland or uninteresting or repulsive.
Lastly, in no way I am saying I am an expert on this, everything above is purely my observation and my perception.
Have fun!
A Restless Mind With
A Sensitive Heart!