I don't think you can upload it to the forum directly.
There are many image hosting sites, tinypic.com is pretty straight forward as you don't need to have an account with them.
You can use MS Paint to resize:
Right click on the image file in the File Explorer, select Open With, Paint.
Select main menu item Image, Stretch/Skew ... Change the Horizontal and Vertical percentages to a percentage less than 100. ...
Select the main menu item File >> Save As to save the resized image.