Hi
you can run randomforest in R , using the randomForest library or alternatively if you prefer you can download the rattle GUI (rattle.togaware.com), your shapefile containing your training data should have the values from your tiff file extracted then you can use the dbf file in rattle. there is a tutorial on the togaware webpage. There is also a library that creates maps based on your RF analysis called modelmap
Recently, I came across a stand alone IDL based remote sensing software called EnMAP-Box that allows you to carry out a random forest classification/regression , you check out the software at Environmental Mapping and Analysis Program
Documentation also on the website
The paper below explains the use of the software
imageRF – A user-oriented implementation for remote sensing image analysis with Random Forests
Environmental Modelling & Software, Volume 35, July 2012, Pages 192-193
Björn Waske, Sebastian van der Linden, Carsten Oldenburg, Benjamin Jakimow, Andreas Rabe, Patrick Hostert
or you can try--> openmodeller.sourceforge.net
Thanks