Instructions
Take a screen shot of your character wearing the headgear you want to customize by pressing "Print Screen" (alternatively, "Prt Sc") on your keyboard. You can also use sprites found on fansites such as the RO Item Database.
Open your image in the photo-editing program of your choice.
Remove the background from the screen shot so you are left only with your headgear. Select the "magic wand" tool (or equivalent) and click on the headgear until you have selected the graphic in its entirety. Lower the tolerance if the magic want selects more than just the headgear. The lower the number in this field, the less likely the tool will select areas outside of your headgear.
Copy the selected headgear by selecting "Edit" and "Copy." Alternatively, you can press "Ctrl" and "C" simultaneously on your keyboard.
Go to "File" and "New" to open a new image canvas.
Select "Transparent" for the background and click "OK."
Paste your headgear image onto the new canvas by selecting "Edit" and "Paste" from the menu or pressing "Ctrl" and "V" simultaneously.
Alter the image as desired. To change its color, select "Image" and "Adjustments" and click "Hue/Saturation." To decrease or increase its size, select "Edit" and "Transform" and click "Scale." You can also open up multiple headgear on the same canvas to mix and match parts.
Save your custom headgear as a GIF or PNG file to maintain the background's transparency.
Use your new customized headgear on graphic images, such "Ragnarok Online" forum signatures.