i followed your advise and renamed the image folders with _old extension forcing the apps to recreate it's image folder. Now it's working much better/faster. Probably my "slowing" problem was the existence of both images (big and small same time) and the program needed to decide which to load).
Apparently, it was the problem, but I just checked my code and I don't understand how having both sizes could slow down the program. The program uses the small image if it finds it on the disk, otherwise it looks if there's the big image.
i can't delete just the small or the big images, that's means a lot of browsing and deleting operation which i don't need.
It's easy to delete only the big images. You ask Windows explorer to search for *_b.jpg, and you delete just those files.
but now i can delete the images_old folder because they are recreated as supposed to be from the beginning.
With my advice just above, you won't have to download again all the small images that you already have.
UPDATE: maybe the small files could not be deleted (and maybe they're slower to load) because they have the attribute "Read-only". You could right click on one of those files, select "properties" and see if "read-only" is checked. If that's the case, it's possible to remove this attribute recursively to all the files of an entire folder, I can explain you how if you need to.