Shrinking a digital photo involves reducing its dimensions and/or resolution. This can be accomplished quite easily by using IrfanView, the leading freeware image editor.

On Unix/Linux platforms, use the free software image manipulation utilities from ImageMagick.

[edit] Steps

Download and install IrfanView.
Run IrfanView. Click File > Open.
Browse to the photo you wish to shrink.
Left-click the name of the photo to select it.
Click the Open button. The photo will appear in the IrfanView window.
To reduce the dimensions of the photo, click Image > Resize/Resample.
Select the options you prefer and click OK.
To reduce the quality of a .JPG image without changing its dimensions, click File > Save As and select JPG - JPEG Files from the drop-down list. Click the Options button and use the slide bar to select a lower image quality. This will reduce the disk space used by the photo.
After altering the image, click File > Save As and select a new file name. Click the Save button to create the new image.

[edit] ON UNIX/LINUX PLATFORMS

Download and install the "convert" utility from ImageMagick .
Example usage: convert -geometry x100 SOURCEFILENAME OUTPUTFILENAME
This creates a new image file of height 100 pixels (good for thumbnails).

[edit] Tips

If you reduce the quality of an image too drastically, simply re-open the original image and try again. This time, save it at a higher quality. You may have to experiment a bit to find an acceptable compromise.
Images over 500 pixels wide will be truncated in wikiHow, thumbs next to text should be 250 pixels wide
If you use a type of digital camera that uses a proprietary picture format, such as RAW or NEF, you will need the add-ins file for IrfanView.