If you are looking for a free alternative to Photoshop look at
Gimp
http://gimp.lisanet.de/Website/Download.html
You can't just resize an image using Preview.
Resizing an image will change its dimensions, but not the DPI/resolution.
Images on the Internet are usually at computer screen resolution of 72 DPI. Some images in the web might be at 150 DPI depending on the type of website it is.
Just resizing an image will make the image worst as you are only making the available pixels in the image larger and more spread out causing an even blurrier image.
You need an image editor that will increase the number of pixels in an image as you increase the size.
This is called interpolation. You are adding artificial pixels to an image to boost its DPI resolution.
These pixels are created from the surrounding existing pixels in an image.
Done correctly, you can get a pretty sharp image from a low DPI image.
Done incorrectly, you can make the image very ugly, blurry and look very out of focus.
You minimize this effect by increasing the DPI of an image in small increments. I use 50 DPI increments.
Some users use even smaller DPI increments like 25 DPI or smaller.
I have a certain amount of impatience, so I use a larger upscale amount. I still get pretty good results that are more than just acceptable. Some user are just more particular about how much detail and sharpness they want their final image to be. I feel increasing the resolution of an image to 300-350 DPI using 50 DPI increments is a good compromise. My own user preference.