lets say you have a huge image file that you want to condense before uploading it to a share file.

most of the time, people have an application installed (which comes with Windows) called "Paint". (usually in the 'Accessories' part of the start menu).

Start Paint
{pick a file, graphic loads}
"select all"
{enter in 50% horizontal and 50% vertical}
{make the numbers half of what they currently are.}

that makes the image and file half the size it was. you can adjust what % you want.

there are other programs that do this easier, but this paint program almost everyone already has on their system.

have fun.