I will be showing you how to do it on Adobe Photoshop CS, however will add Microsoft Paint and maybe a few others when time permits, for those not lucky enough to have Photoshop CS installed on their computer.
So lets begin:
Adobe Photoshop CS
Photoshop (PS) is definately one of the most common and user-friendly graphical editoring tools available to consumers and businesses. Its features are pretty advanced as well, once you delve into it. I have also taken some screenshots for the visually inclined users among us (lazy ones).
Step 1
Select the desired image you would like to resize by clicking on the File menu at the top left of Photoshop and then clicking Open.
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Highlight the image you would like to use, then select the Open button on this screen.
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For this example, I chose a 1024x768 image - Aston Martin!
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Step 2
To resize the image, click on the Image menu, again at the top of the screen, then select the Image Size option (this can also be done using the shortcut ALT+SHFT+CTRL+I).
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On the next screen you are presented with, you will need to tell the program what kind of dimensions you would like the image to be. I generally select the Height and Width first, and generally make the drop down boxes 'Pixels' instead of any other types of measurement.
The chain looking object next to the drop down boxes is actually directly linked (no pun intended) to the tick box at the bottom of that window that says Constrain Proportions. All this option means when ticked, is that if you change the height or width of the image, it will change the other accordingly, so the shape of the image is kept intact (eg. image size is 60x60 and you change the height to 30, the width will change to 30 as well to keep the square shape).
This is a feature that's user decided, but I generally leave it on, so as not to distort my images.
Once you are happy with the size of the image, click the Ok button to the right hand side.
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Step 3
Now we need to save the image so it's on your computer. To do this, go to the File menu again, and select Save As. You need to do a 'save as' if you wish to save the image to a seperate file instead of the originally sized one. The save function in Adobe Photoshop CS is the same as any other program, however you may want to change the filename of the image.
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And we're done.
Your image should now be resized to the correct sizing and you can do what you want with it.
He's the Aston Martin, resized to half its original size, 512x384.
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p.s - if there's anything I forgot to mention, or could add, feel free to PM me.