If you want to use Paint like DDtoys suggested - go

Start
Programs
Paint.

Then open your image in paint , look along the top of the Paint window and go to Image -click it once
Click Stretch / Skew once and it will open a new little window where you can change the size from 100 x 100 % to (say) 50x50%.
Then re-save it in a folder on your desktop and use that new image to upload here.

To get the image to show in my threads ( instead of just a link ) I upload them to Imageshack....once up there ( or photobucket as Greg said ) you can get a 'Direct Link to Image' link, like in the image below....




Take that link and copy and paste that into your thread by using the icon I am about to indicate in the image below




Hope this helps