Images are automatically resized by the forum if they are over 600 pixels wide. Unless you have told it to do otherwise in your settings that is.
I know that, but how do you make it smaller for the organization of your thread? How can you customize it's size?
Then you need an image editing program to resize them. If you don't have photoshop or GIMP, Microsoft has an image resizer. Look down he list on the right here: Microsoft PowerToys for Windows XP