Try using this tool: Image Resizer direct from Microsoft. Install it then you can just right click your image files and choose "resize picture" and it'd do the job for you. You can even choose multiple images for resizing too.
Correction image resolution here is usually 800x600 as Moogle said. I usually post using that size. I guess 500k is the maximum file size that can be attached? I'm not sure about external links though. That could be different.