By going to Image > Image size after opening an image, I made sure the resolution was set to 300 dpi (industry standard for print), and the measurements were set to A5 (postcard size). I also made sure the document was using a CMYK colour mode by going to Image > Image mode.
I then decreased the saturation to -100 by creating a Hue & Saturation Adjustment layer.
I added a colour tint by adding a Solid Colour adjustment layer and setting the blending mode of this layer to 'Overlay'.
CREATING A PATTERN:
I then duplicated this image, rotated it vertically and positioned it directly underneath the original.
Using the pen tool, I created a path around the building and extracted this by going to Paths > Load path as selection and then simply deleting this.
I then duplicated this once more but instead flipped the image horizontally.
I then cropped the image to its exact proportions, removed any unwanted background and went to 'Edit > Define Pattern' in order to transform the image into a pattern.
For the reverse side of the postcard, I opened another A5 document at 300 dpi, created a new layer and filled it with the colour white. By going to the Layer Style option menu, I am able to add a pattern overlay and decide on a scale I wish to use: