You'll need some sort of image editing programme, Photoshop, Paintshop Pro or similar. Whichever app you use there should be an option to change image size. Reduce it to the size you want, and if it's for the web make the resolution 72dpi and save it as a new file. Use .jpg as the file type and a compression level of 8 should give good quality and small file size. - Baz