Often times the best photos to use for your website will be a large high resolution image that will need to be sized down. 
If you do not have access to professional photo editing software such as Photoshop. There is a free software alternative for basic photography edits call PicMonkey. 
The video below walks through re-sizing a photo on picmonkey. 
1. Go to PicMonkey.com 
2. Select Edit and then upload a photo using one of the options 
3. From the menu on the right at the bottom select Resize 
4. Maintain the proportions so the photo doesn't get skewed 
5. Put the dimensions you'd like the photo to be. With the exception of sliders and header photos no image on your website should be larger than 1000 px any direction. 6. Select save and then your photo will be saved to your computer and ready to be uploaded to your website.