> Often I see that some .jpg files are re transferred with 
> a   .JPG extension,these are files I've already 
> downloaded:is there a way to make HTT assume that these 
> files are the same though or is there a specific filter 
> string which could still let me download missing jpg file?
No ; if the server is not able to send "not modified" 
responses, unfortunately httrack won't be able to detect 
that the files were not modfified

> Last topic:I see that HTT is getting slower each time I 
re 
> scan the files(note:no crashes, wrong interruptions or 
> else).

Can you try to add this switch ? (among with urls you 
type) :
--fast-engine

This should speed up the whole process