If you want to go the photoshop route:
1.) Open the image in ps
2.) Go to Image> Image size (or apple key option I for mac)
3.) In the height and width section, you can change it from pixel to percent, change to percent.
4.) Make sure the constraint proportions options is check at the bottom.
5.) change to any percent you want, 95%, 75%, or what ever suits you.
Remember, you can't make images bigger, or they will pixelate.
Also, when you save it, click "save for web" and it will automatically save it as a jpeg or gif file, instead of a photoshop formate.
The process sounds hard, but do it a few time, and you will be able to do it with your eyes closed. This works great if you have a lot of pictures to resize, but if you only have one or two, I would stick to the other methods.
Let me know if you have any other questions.
Regards,
~Johnny