wget not to download images already downloaded..

[Hopeless]

Hi,

I have two folders and in one folder is all the forum images i added manually to my .wgetrc file by doing
ls > test.txt yeah i know took a while addin em...but hey i'm new to linux...

i have another folder called mainuploadsfolder with everyones folder and inside there are images already downloaded, how can i direct wget to skip all images already downloaded...? do i keep doing a
ls > test.txt ?

Many thanks !!