When you open an image file in Windows 7 it opens in Windows Photo Viewer by default. It can be used to view and rotate images but nothing else, so you need a better piece of software if you intend to crop, resize, adjust or otherwise edit your photos.
IrfanView does all this. It is a piece of freeware that I have used for as long as I have used the Opera browser and the NoteTab text editor, ten years or so. The user interface is minimalistic and the program is controlled using the menus or the shortcut keys. IrfanView can be used to batch process an entire folder full of images or just one image at a time. It can read dozens of file formats and write an impressive number of formats too. There are also lots of plugins for it bringing in some extra functionality. They are really easy to install because they are provided as one big installer file from the IrfanView homepage. What's also very important for netbook users is that IrfanView is a lightweight and fast program.
Hint: If you work with images that have transparency, set the IrfanView background color to pink (Options | Properties/Settings... | Viewing | Main window color). Then you will know exactly where the transparency starts and where it ends, as pink is featured in really few pictures, unlike black and white.