I should have clarified my response on this one a bit.
You're talking about three things here:
- Making all images the same size.
- Inserting a watermark.
- Removing the background.
As others have said, the first two are quick easy and can be perfectly automated. As others have pointed out, if you are working with the original JPG, then any changes to the image will result in a reduction of quality. Exactly how great that reduction depends on how great the change is. If you have the original images, you should use those instead.
The third part is the one that could take a year. Each image is likely a different shape, etc., so if they aren't on a standard background (like a solid color), it's very difficult (or impossible) to automate something like that. If they are all on a solid colored background, then batching that would be possible (though likely require some hand touch-ups).
Also, you should note if by "removing the background" you mean making it transparent, then you will have to save them as PNGs (JPGs don't support transparency).