A few things before you start.
Choose the correct image for your purposes.
A panoramic-style image in landscape orientation works best for a Facebook header. Trying to force an image of your face into a long horizontal shape is obviously not going to work. On the other hand, if you're looking for a Facebook profile image, then perhaps a portrait image is preferable.
Let's use an example image of some neuronal synapses, mainly because this is a neuroscience site, but also because the image is correctly oriented.
Know your intended purpose
Are you trying to increase or decrease the image size?
Cropping an image is a good way of achieving an exact image size, changing the aspect ratio (shape) of the image, or just decreasing the image size. You must use an image with a greater number of pixels to start with than what you need for the final image, both in length and height.
Increasing the number of pixels is possible but more complicated, and beyond the scope of this particular post.
Are you resizing to achieve a particular image size, such as for a Facebook header?
Facebook banners are 851 x 314 pixels. Give yourself the best opportunity for success by using an image of this exact size. Any other sized image will need to be repositioned, and possibly even resized by Facebook software, to fit these requirements.
Simple Microsoft Office software will do the job
There are plenty of sophisticated and powerful image processing software packages out there. Photoshop is a prime example. You don't need to buy this software or know how it works for this simple resizing process.
Let's get started.
The Procedure
How many pixels does your image have?
Open your well-chosen image in Microsoft Office (Right click on image > open with > Microsoft Office 2010).
Click on 'Edit Pictures' on the toolbar.
Under 'Change Picture Size' on the right menu bar, click 'Resize'.
The pixel resolution of your image will appear. In this case, the image is 3664 x 2880 pixels.
What is the aspect ratio of your intended image compared to your original image?
The aspect ratio refers the ratio between the image length and image height. This gives a measure of the image 'shape': an aspect ratio of 1 indicates a square, whereas less than or greater than one indicates a vertical (portrait) or horizontal (landscape) rectangle shape respectively.
We want to manipulate our image into the same 'shape' as the required Facebook image. The aspect ratio of Facebook's header is 851 / 314 = 2.7, whereas this example image is 3664 / 2880 = 1.27. Therefore, we must change the image 'shape' by cropping one of the sides.
Crop the image to achieve the intended aspect ratio:
Because the aspect ratio of our desired facebook header is >1 then we know we are creating a rectangle in landscape orientation. Therefore, we should keep the length of our image the same, but crop the height to achieve the desired aspect ratio of 2.7.
Use simple maths to calculate the new image height: 3664 / 1352 = 2.7
Again, click on 'Edit Pictures' on the toolbar.
Under 'Edit using these tools' on the right menu bar, click 'Crop'.
The 'original' and 'new' (cropped) dimensions are listed under under 'Picture Dimensions' on the right toolbar
Pull down the black bar at the top of the image until the selected region is the correct size (in this case, 3664 x 1352 pixels).
Move the selection into the correct position to retain the part of the image you are interested in.
Click OK
Resize the image to achieve the intended pixel resolution:
Again, click on 'Edit Pictures' on the toolbar.
Under 'Change Picture Size' on the right menu bar, click 'Resize'.
Under 'Resize settings' select 'Custom width x height'
Enter the desired width and height respectively (851 x 315 pixels)
Click OK
Make sure to save your modified image with a different name.
Make sure that you understand pixel resolution!
A digital image is made up of a given number of pixels. However, unlike a printed photograph, the digital image has no fixed size or resolution. You can resize the image as desired to cover a larger or smaller area and in doing this you will be effectively changing the resolution.
If you make the image smaller, the same number of pixels will be squished into a smaller area. Your eye will not be able to distinguish individual pixels, but rather continuous shapes, giving the image more clarity. Conversely, if you make the image larger, you are stretching the same number of pixels over this larger area. This will make each individual pixel visible to the eye, and give the image a discontinuous, blurry appearance.
What NOT to do:
Do not simply crop your image to the intended pixel size in the first step. This will effectively stretch out and zoom into your image so that each pixel is visible, giving you a poorly defined, low resolution image.
Instead, you want to maximise the number of pixels (within reason) that lie within a given space. By manipulating your image to the correct aspect ratio (shape) first with minimal cropping, and then resizing, you are retaining more pixels for a given space. This will maintain the resolution of your final resized image.
Now you can try making a different image for your Google+ or Twitter headers.
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