There are probably plenty of online image resizers. But I just use MS Paint on my local machine. Here are the steps I follow to resize an image so it can be uploaded to fordbarn:
1. Right-click on the image file
2. Choose Open with... Paint
3. Choose File, Save As... to save to a different filename (which you'll upload later)
4. Click Resize
5. Enter the percentage of resize, click OK
6. File, Save
Fizzy images happen when you have resized or saved a file multiple times. Make sure to start with a large original file that hasn't been digitally manipulated.