Hi icelliate! :-)

I recommend this:

Free Image Resize Software Import organize resize photos- VSO

Is a very simple software that you install on your PC and, then , when you click with the right button over an image it let's you convert it to any format/size you want.

Here it's how to use it: How to Resize Image for Web-How to Resize Digital Image-VSO Software

Greetings :-)