There's a few ways, here's just one option:
- Right-click the picture file and choose Open With > Paint.
- Click Resize on the toolbar at the top (near the left edge)
- To make the picture half the size, put in 50 (the other other box will go to 50% also), then click OK.
- Now to save the now-smaller picture, click File > Save As and save it as a jpeg file picture (either replacing itself, or with a new name so you can keep both the large and new smaller picture).