hanafi, I use irfanview for most of my imaging needs.
Here is a screenshot of selecting the "Image" option (3rd to the right). Then selecting "Resize/Resample" from the dropdown list. You can also press the keys "Ctrl+R" to get there.

Make sure that you have checked the box next to "Preserve Aspect Ratio (proportional) when you modify your image.
Another option is to use the "NEW! Free Image Optimizer" that is just below the "Upload Image" button that you can see when you "Reply to Topic".