Never used JQuiry or Javascript

Never used JQuiry or Javascript

I have built websites for several years but have not used java script other than ready made scrips for things like guest books
Because of the number of different resolutions used, I now need to be able to swap photos (or resize)
according to the resolution the visitor is using, I believe this may be possible with jquery but I don't really know
where to start , some advise would be appreciated
Thanks