The Web-based Image Resizer will let you upload an image and then download a resized version. To make it quick, low-quality versions are shown for a preview, but when you choose to download, a high-quality resize algorithm is used to give you the best possible image. You can download JPEG or PNG. If you don't want a standard size, you can choose to do a custom resize and either provide the exact pixel size you want or use a slider to choose the percentage you want. Use the app or get the source code.