Interpol said the images of the suspects' faces were reconstructed from remains found at the sites of the attacks; a restaurant and rugby club where people had gathered to watch the World Cup final.
Kale Kayihura, the inspector general of Uganda's police, told a news conference in Kampala that he had no doubts that suicide bombers were responsible.
"These attacks were carried out by suicide bombers. The evidence is overwhelming ... two heads have not been claimed, neither have they been identified. It can't be a coincidence," he said.
Kayihura said that the reconstructions suggested that one of the bombers was of Somali origin and the other a black African of undetermined origin.
Al-Shabab said it organised the attacks to avenge the killing of civilians by African Union peacekeepers. Ugandan forces form the backbone of the 6,100-strong contingent protecting the UN-backed transitional government in Somalia.
The group is accused of having links to al-Qaeda and is classified as a "terrorist" organisation by the United States.
A number of arrests have been made since the attacks, according to Ugandan police.
"We have arrested more than 20 people arrested, some of whom are foreigners, including Pakistanis," Kayihura said.
"They are being questioned ... They have to explain themselves."
Interpol, which helps co-ordinate police investigations and information-sharing between member countries, said it had sent a team to Uganda to support police there in their investigations.