Solis,
In Photoshop, go to 'Image' > 'Resize' and a resizing window will open. Use pixels. About 800pixels wide is suitable for most sites.
To help get it down to under 200kb, go to 'Save for web'.
Choose JPEG, and use the slider on the right to adjust the kb level (bottom left) to under 200kb.
Here are screenshots I just did for you
