This is what i do when I need to re-size a picture quick from my work PC (Windows):
o resize a picture using Paint:
Open Paint by clicking the Start button , clicking All Programs, clicking Accessories, and then clicking Paint.
Click the Paint button , click Open, click the picture you want to resize, and then click Open.
On the Home tab, in the Image group, click Resize.
In the Resize and Skew dialog box, select the Maintain aspect ratio check box so that the resized picture will have the same aspect ratio as the original picture.
If the Maintain aspect ratio check box is selected, you only need to enter the horizontal value (width) or vertical value (height). The other box in the Resize area is updated automatically.
Do one of the following in the Resize area, and then click OK:
To resize your picture by a certain percentage, click Percentage, and then enter a percentage to reduce the width by in the Horizontal box or a percentage to reduce the height by in the Vertical box.
To resize the picture so it's a specific size, click Pixels, and then enter a new width in the Horizontal box or new height in the Vertical box.
Click the Paint button , point to Save as, and then click the picture file type for the resized image.
Type a new file name in the File name box, and then click Save.