Called 'Baby Doe" by  Boston police, the four-year-old girl was found by a woman walking her dog on 25 June.
The National Center for Missing and Exploited Children created the image in the hope that someone who knew her will come forward.
She was found with a zebra-print blanket and wore polka-dot leggings.
The centre posted an updated version of the image to add a pair of earrings. She is believed to be either white or Hispanic, weighed about 30lbs (13.6kg) and stood 3.5ft (1m) tall.
Police believe she died not long before her body was discovered inside a large plastic bag on Deer Island in Boston harbour.
A post-mortem examination hasn't yet determined the cause of death, according to the Boston Globe newspaper.
Officials said the case has received a massive response on social media and drawn thousands of new visitors to government websites.
"Every now and then we have a case like this, and that probably has a lot to do with the age of the child, the circumstances in which the child was found," Suffolk District Attorney Daniel Conley told the Washington Post.
"Sometimes it touches the hearts of many and it motivates the public to get motivated to help."

In an unexpected decision, the BOJ eased monetary policy on worries that lower oil prices would affect consumer prices.
The central bank will increase the pace at which it expands base money to 80 trillion yen ($726bn; Â£454bn) a year, instead of 60-70 trillion yen.
The benchmark Nikkei 225 index closed up 4.8% at 16,413.76 after the news.
It had hit a seven-year high of 16,455.84 in mid-afternoon trade after rising by more than 5%. The dollar rose 1.2% to 110.68 yen, the highest since January 2008.
Several pieces of data out of Japan in the morning showing a weaker economy also failed to dent investor sentiment.
The country's annual core consumer inflation eased for a second straight month in September, while its jobless rate rose in the same period. Household spending also fell more than expected last month, adding to evidence that the central bank would probably maintain its massive stimulus programme.
Asian shares, meanwhile, were already up after positive economic data from the US showed strength in the world's largest economy.
Data showed the US economy grew at an annual rate of 3.5% in the third quarter. The news came a day after the Federal Reserve announced the end of its monetary stimulus programme.
Greater China shares followed the uptrend, with Hong Kong's Hang Seng index closing up 1.25% at 23,998.06 points.
On the mainland, the Shanghai Composite closed up 1.3% at 2,420.9 points.
In Australia, the benchmark S&P/ASX 200 closed up 0.9% at 5,526.60 points, following Wall Street's gains overnight.
Shares in heavyweight miners BHP Billiton and Rio Tinto rose 0.8% and 1.8% respectively.
In South Korea, the Kospi was up 0.3% at 1,964.43 points.

The 6ft 4ins midfielder was recommended to the club by former Walsall and Bermuda striker Kyle Lightbourne.
The 18-year-old has been training with the League One Saddlers since appearing in a development squad match in April.
"I've loved it here, the way the club has treated me," he said. "And how the players have welcomed me straight in."
Butterfield will initially work under the vastly experienced John Ward, Walsall's professional development coach.
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A computer-generated image of a girl who was found dead has been shared millions of times online as US police search for clues about how she died.
[NEXT_CONCEPT]
Japan led Asian shares higher after the Bank of Japan (BOJ) surprised markets by announcing more stimulus measures.
[NEXT_CONCEPT]
Walsall have completed the signing of Bermudian teenager Kacy Milan Butterfield on a one-year deal, subject to receiving international clearance.