I use Gimp
http://www.gimp.org/ It's free.
Open the image in Gimp.
Click on the word (image) in the top menu.
Go down to (scale image). Click on it
The scale image box will open.
In the image size boxes take the width or Height which ever the larger number.
change that number to I think its 500 pixels.
then go to the bottom of the box and hit scale.
Now here is the important part, you will want to save your image in a form that this forum will accept.
I Personally like PNG images. But that's me.
You'll have to go to the file menu.
Scroll down to export, click on it.
in the export image box pick a folder then go down to (export all images) in the bottom right corner. Click on the down arrow.
All the different formats will pop up. Pick a format. I'll just use the { PNG image (*.png) } format for an example. Click on it.
Then go to the name box at the top. Name your image lets say- Image.png. just that way with the .png after it.
then go to the bottom hit export.
A properties box will open. Check what ever properties you want to use.
then click on export again.
you have your image. A very clean image.