Make sure the photo you choose is square and clearly visible at the small size. Many if not most Twitter users are using Twitter on their phones. Your photo is going to be the size of a gnat on there. It has to be recognizable at that tiny size. If you want the image to show you as a person, don't show a full body image - show just a close-up on the face.
Note that the images show up with slightly rounded corners. Twitter does that for you. You simply load up a square.
Make sure the photo is well taken. If the photo is fuzzy or out of focus it is going to give a poor impression to your readers. People really do care about those sorts of issues. People react very strongly to images.
Remember, your icon shows up next to EVERY POST YOU MAKE. That is a huge amount of promotion. It needs to do a good job.
I would NOT use a stock photo or generic image. Again, this is all about branding. Branding your name and words with a stock photo or image is anti-branding :) It will do you no good in the long run. People are keyed to visual images and if you are building your reputation as a writer you can use every boost you can get! Definitely use your photo, and make it the best photo you can get. It is far better to use a photo and later upgrade to a "better photo" than to have a generic, meaningless image that will not provide any value to you.
Whatever you do, don't stay with the initial "egg." This is a sign of a spammer and many people automatically block egg-accounts. Make sure you replace that egg as soon as possible.
Here is a pair of examples from organizations in the Worcester area. The first is the Worcester Art Museum. They chose to go with their abbreviation, WAM. It's simple, easy, and shows up clearly on a tiny cube.
In comparison, the Worcester Cultural Council found an interesting way to fit all the letters of Worcester into the cube. The tiny letters below will be a blur on a smartphone but anyone clicking through to the larger version can read it.
When I was setting up a page for a site about Worcester, I first went with an iconic landmark. It was an interesting idea - but at the tiny size this turned into a meaningless blur. That wasn't good.
Next, I tried the words with blue-on-yellow. But at the small size this turned into illegible fuzz.
I tried a thinner style of letter. This was better at the small size, but still hard to read.
So I changed the blue letters to black. Voila, suddenly it became much easier to read.
Still, it didn't seem to have much personality. My point for the Worcester MA site was to promote local events and fun. So I worked on making the letters in WORCESTER MA fit into a clock face. Voila! That's what I wanted and it worked on large and small sizes.
It's fine to practice and test different options to see what works best both on PC screens and on smartphones. If you don't have a smartphone, enlist a friend to lend a hand with testing. You need that image to be recognizable and clear.
Once you settle on an image, it's best to stick with it. People will come to associate your image with you as an account. They'll look for that image and when tweets come through their stream they'll recognize it. If you then randomly change your image every week, they'll lose that ability. They will not have as strong a connection with your posts. So while it's good to experiment until you get a great image that works well at small sizes, once you find one, stick with it.
Building Traffic for your Twitter Account
Followers and Twitter's Value
What is a Twitter Hashtag?
Maintaining Multiple Twitter Accounts
Choosing a Twitter Icon
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What is a Twitter Hashtag?
Twitter for Authors
Twitter Promotion Tips
Twitter Next Step
Twitter 2000 Follower Limit
TweetDeck - Free App for Twitter
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FollowerFilter - Great Free Follower Management Tools
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BellaOnline Twitter Template Design
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Twitter Business and Pleasure by Erwin Calamer
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