If it's too big you can reduce it using Windows Paint program. Instructions follow:
Open the image in Paint.
Then go to Home tab, Image Group and then go to Resize
Here you will get option to resize the image by Pixels and by Percentage. You can choose any one of them.
If you choose pixels then pixel size of original image are displayed in the boxes below. You can reduce the pixels as per your wish. Make sure that "Maintain Aspect Ratio" is checked. If its not checked then image width/height ratio will be disturbed and you might not like it.
You can also choose to reduce the image size by percentage. But make sure that "Maintain Aspect Ratio" is checked.
Once the pixels are reduce, then you can save the file from File menu. If you choose the "Save" option from "File" menu, then same image file be replaced with new image of lesser size. But better option will be to choose "Save As" option from file menu, as it will keep both files, so that you can compare them later.
Or you can do like mrbb said and upload to a pic file host such as photobucket and then just post the image URL and walah!