For free you can use a website to do all sorts of editing without having to install anything.
Found out about this website in College from Art Professor who pointed the class to use it.
https://www194.lunapic.com/editor/Basically its as easy as uploading a image to the websites web page session that you have and do whatever you want with it and when done your able to download the edited version of it for free.
I am guessing that when you mean pixel number your meaning the X,Y dimensions of the image which can be done in resizing an image.