Forum Moderators: open
You need eithr an image program or a wysiwyg editor that allows you to resize images THEN resample them at their new size. Merely resizing without resampling actually won't change the image's overall size, especially if you cannot define the height and width in a page, something you can't do if it's a background image. If you don't have any editor, do a search. There are several free ones online.
Marshall
W3C Validator - HTML [validator.w3.org]
W3C Validator - CSS [jigsaw.w3.org]