I think you pointed this out the other day, and I, too, think the statement made by the mayor on Wednesday is rather telling:
"Mayor Bill Peduto said Wednesday that authorities have followed up on every lead theyve received, but nothing has yielded results.
'We know all of the facts about where he was that evening, who he was with that evening, the condition he was in that evening, where he was traveling that evening,' Peduto said. '
And then, once he gets to that bridge, it all disappears.'" (BBM)
http://www.wtae.com/article/dakota-james-mother-criticizes-police-no-evidence-of-abduction/9081075
One possibility I can think of is that LE has an image of Dakota getting on the bridge, but may not have released it because the family has not agreed to it ("Family viewed the new video from the Cultural Trust and agreed that the still photo of Dakota in Katz Plaza should be released to the media"; there may be other photos which the family has not agreed to releasing).
http://triblive.com/local/allegheny/12004988-74/james-police-duquesne
Maybe it isn't a very clear image, and the family has doubts as to it really is Dakota. Further, if it isn't very clear, it wouldn't make much sense to release it to the public for the purpose of jogging their memory.