If you want the thumbnails to work as designed then you do nothing.
If there is some other issue here like you recieve errors uploading you might want to pass that error along so we can assist.
EXAMPLE
http://www.ciclism.ro/forums/classif...stadt_bike.jpg
shows fine
http://www.ciclism.ro/forums/classif...stadt_bike.jpg
does not show
Therefore when you upload what is the error you recieve? did you make sure the data directory at every level is set to 777 permissions that means the numeric directories beneath data as well as the thumbs large directories beneath the numeric directories?
data 777
--2 777
--thumbs 777
--large 777