This one is for you Solilo :) I am only sorry that I cannot give you any pictures that I myself have taken :( All of the pictures have been sourced from the web, and all of them belong to their owners :)
Disclaimer:
For all : This post is way too long, because I wanted to include as much as I could :) and because I simply do not know how to organize :P! Way too many words, and way too much of rambling, so I do hope you will actually understand something. If you do, tell me. If you don't, please smile nicely, or look the other way :P :P!
Here goes:

( The iconic picture associated with Thrissur Pooram, which shows the Devi of Paramekkavu Temple-in the background- being taken out for her procession. 15 elephants, the musicians in front, and a sea of people to welcome her and accompany her!]
http://maps.google.com/maps/ms?ie=UTF8&hl=en&t=h&msa=0&msid=114038809751246788218.000440d8cff499fc85a7c&ll=10.526843,76.215742&spn=0.009493,0.013819&z=16
( That is the link to the satellite image of Thrissur, for you to understand the layout :) I do not know how to get the map here :( )
An Introduction to Thrissur, for it is necessary to know the geography in order to understand the Pooram :)
You see, Thrissur has been rather ingeneously designed and built. The central [now town area] area is actually a sort of rough circular area, around tiny hillock, which, once upn a time was a veritable forest of teak trees, and so was called Thekkinkkad [forest of teaks]. Later, it came to be known as Thekkinkkad Maidanam, after many of the trees were felled or removed for expansion. In the midst of the Thekkinkkad Maidanam sits Lord Shiva, known there as Vaddakkumnnathan [Lord of the North - Vadakku means North, Nathan means Lord]. The area of the Maidanam, in front of the West Gopuram [gate] of the temple, which is the front of the temple, is called the Sreemoolasthaanam, where all the dieties from the different temples come, to pay their respects to the Lord, of Thrissur. In fact Thrissur has been named after Lord Shiva... the full name being Thrishivaperur... the town in the name of Lord Shiva. Thri Shiva, Lord Shiva; peru, name; uru, town :) To continue with a description of the town, (please also view the map, whose link is given at the top of this post), the Maidanam is encircled by a single road and it is called the Swaraj Round. Much like the Connaught Circus in Delhi, which has radial roads. The Krishna Temple is down the Shoranur road, north west of the Maidanam, and the Devi Temple is East of the Maidanam. The Devis from both the temples, basically meet at the Sreemoolasthanam. That is the gist of it. Now to the details :)
And the rest of it, if you have the time, the inclination and the energy :), you shall find here, on my wordpress blog:
http://ushus.wordpress.com/2009/05/19/%E0%B4%A4%E0%B5%83%E0%B4%B6%E0%B5%8D%E0%B4%B6%E0%B5%82%E0%B5%BC-%E0%B4%AA%E0%B5%82%E0%B4%B0%E0%B4%82-thrissur-pooram/