When a picture has been added to your form, click the picture to display its "handles" (the small black dots around the edge of the picture). By clicking and dragging these handles, the picture can be resized as required. Note that the picture will retain its aspect ratio when resized. Enlarging your picture may reduce the quality of the image, so ensure the resolution of your image is adequate before adding it to your form.
Ideally, it is best to use an image that is already the correct size, to avoid any image artifacts/dithering which can be caused by resizing. These artifacts can occur when enlarging and reducing, but they may be particularly apparent if you use a small image and enlarge it on the form.