Max resolution of new and resizing down the existing ones

First of all: SORRY IF THIS TOPIC WOULD BE JUST A COPYCAT OF A BUNCH OF SIMILAR OTHERS. I tried looking with Google, but found just shreds of informations, but not what exactly fits my needs.

I have a lot of albums filled with a lot of picture, all uploaded at the original size (from 1024x768 up to 1856x1392). What I want is to find a way to scale the existing ones down to max 1024x768 ALL AT THE SAME TIME; I know how to resize them one by one, in the single photo properties; but it would take me MONTHS of work to do manually for each one!

And, for the coming uploads, where and how to set the maximum resolution showable, despite the original size?

I'm thinking about selling my pictures, and I don't want to upload them with a large resolution; but I don't want to resize them manually!

Thanks for every answer you would give to me!

Francesco

P.s.: feel free to erase the thread if it's the 10.000th with the same request; just, in case, advise me with a pm. Thanks mod.