This software can reduce image weight without loosing quality (you won't see it). It's called Alice's Biscuit and it's free.
This is very useful for background images for your beatmap or any wallpaper or huge images you have to upload.
1) Drag and drop your image(s) you want to reduce.
2) Select "Taille réelle" (in order to not change the resolution of the image).
3) Clic on "Enregistrer" button and choose the final path.
You're done ! Sorry for my english, and I don't know if this tutorial is in the right section of the forum. Hope it will help some of you.
Website : http://www.alicesbiscuit.com
Download : http://www.alicesbiscuit.com/telechargement-reussi.php