The recommemded width of the image is 500 or 600 pixels.
Large images take a little bit longer to load and may not look correct on all monitor sizes.
Windows XP, use Image Resizer Power toy from Microsoft. You can download it here.
After installing Image Resizer, right click on the image and resize.
Ubuntu, use GIMP. From Top Task bar, Choose Applications->Graphics->GIMP Image Editor. Open the image you want to resize in GIMP. Choose Image->Scale from the GIMP Menu. Change the width of the image. Click Save.
Window 7, Right Click the image you want to resize, choose open with paint, click Resize icon, change the dimensions, click Save.