> can HTT be made to download only images of type .gif 
> and .jpg (for example) that have the size set in pixels 
to 
> 1600px x 1200px?? is it possible?
No - you can only filter with rules based on link names or 
size (note: the link size limit is a hack - and will cause 
empty images to be generated) ; such as
+*.gif -*.gif*[<10] +*foo.gif*[>50]

(accept all gif images, except images smaller than 10kb -- 
except images named *foo bigger than 50K)