Click on File; New... and in the dialogue box that
opens set Width to 1024 pixels; Height to 768 pixels; Resolution to 240
ppi; Color Mode to RGB Color 16 bit and Background Contents to
Background Color. Click on OK.
You will now have a 1024x768 blank image. Let's
call this the Background Image. Minimise this window.
Next, open the first of your three picture images.
Click on the Crop Tool and in the
tool attributes in the top of the window right click inside the box for
the Height value and select Pixels. Set Width to 342 px and Height to
768 px. Now click on the top left of your image and holding in your
left mouse button drag to the bottom right a marquee (dotted line)
selection. If you do it wrong, just start again drawing from the top
left. To resize your selection, just click and drag any of the handles
in the corners of the selection marquee. Once your selection is the
right size, left click inside it and drag it into the best position
over your image to select what you want to appear in the final image.
Use the arrow keys on your keyboard for fine tuning. Hit Enter to crop
the image. Save this image and minimise it.
Open the second and third of your three images and
following the above workflow in point 3 resize all the images.
You now have four files open in your workspace.
Go to your first picture image and copy it using
CTRL+A to select and CTRL+C to copy.
Go to the Background Image and paste the first
picture into it using CTRL+V. Select the Move Tool and move the image to the far left of the screen
by left clicking inside the image and dragging it.
Repeat as above, copying and pasting the second
picture in the middle and the third picture on the right. Do not worry
if any of the pictures overlap a bit.
Click on the Set Foreground Color icon to switch foreground and
background colours so that the black square is uppermost.
Select the Rectangle Tool and in the Tool Attributes, deselect Anti-alias.
Starting exactly on the edge of your Background Image, drag a black
rectangle onto your picture right along one edge so that it extends
into your image 8 pixels wide (thick). You will find it easier to
achieve an exact measurement if you have the Info Bar open and can see
the pixel size. The Info Bar tab is normally on the top right of your
workspace after the Navigator and Histogram tabs. Do the same with the
other three sides so that your picture now has a black frame all around
it 8 pixels wide.
Do the same vertically down between the left and
middle images creating an 8-pixel border between them, and likewise
between the middle and right images.
Now save this image as a PSD file which retains the
layers and allows you to go back and change the various aspects of the
image later, which you cannot do in a JPEG.
Then, in the top menu, go to Layer; Flatten
Image... and Save As (or Save for Web and Devices) and select the
JPG / JPEG option.
You now have three images each separated by an
8-pixel border - and a very pleasing triptych!