The pictures aren't representative of Tallinn as a whole, certainly not the central parts, but it is probably fair to say that it is more run-down in parts than similar cities in wealthier countries. (Remember that Estonia isn't rich -- GDP per capita is one third that of Belgium, for example.) How much you see of this obviously depends on where you live and where you go.
Also: when looking up stuff online, learn to love Chrome's built-in translation functions, and remember that it (and Google Translate) unfortunately does a better job with Russian than it does Estonian.
Best single Tallinn resource is probably In Your Pocket: http://www.inyourpocket.com/estonia/tallinn
You can pick this up cheaply in the city, download the pdf from their site for free, or get their iPhone app (also free).
Also new and interesting-looking is http://www.likealocalguide.com/tallinn
There is no great definitive online expat forum.
Also: when looking up stuff online, learn to love Chrome's built-in translation functions, and remember that it (and Google Translate) unfortunately does a better job with Russian than it does Estonian.