1. Open the image you want to resize, using "Preview". You can do this by doing a right click on the image and selecting "Open with" and then "Preview".

2. From top menu select "Tools" abd then "Adjust size"

3. Change the size from "cm" to pixels

4. Input either the height or weight and if "Scale proportionally" is checked, you don't need to worry about other dimension. For internet more than 800x600px is not necessary. Press "OK".

5. Press "CMD"+"S" or "File" and "Save" from the top menu to save the image.

That's it!