How to create a white background to a jpg/png? I have a jpg of some logo, but its background is not completely white, more like light-greyish, whereas I would like it to be perfectly white.

I know how to select the background and also know how to turn that background to transparent, but not how turn that background to white.

If it can't be done with Preview, I also have iPhoto. I don't know how to do it there either.

NB: If the thing needs to be converted to a png in the process, that's fine.
I don't know how to do that with Preview.app or iPhoto, but for this purpose you could use an online photo editor. First hit on google here is http://pixlr.com/editor/ - and it does the trick.

(upload the image and you can make it white in several ways, e.g. by pressing B or G after you have set the main color to white, though there wil be more intelligent ways)