Picture Sizes

My digital camera takes pictures at a resolution of 3000 something. When displaying on my PC, different programs do different things. Windows Paintbrush and internet Explorer shows the picture at its large actual size. Windows Picture Viewer and Firefox show the picture at a viewable size.
I post these pics on my website and people can not see them correctly mainly because of using IE.
How can I save my pics, they are in JPG format, to a resolution of 800×600, etc?
Thanks.