Work at the actual size
View the image at postcard scale before judging its detail. Large shapes and a clear focal point survive reduction more reliably than elaborate texture. Leave room for the object to feel balanced in the hand.
Pair picture and message
Write a short message that changes how the picture reads without merely describing it. Compare sincerity, irony, and silence as different ways of connecting word and image.
Consider the back
Address space, writing area, and orientation affect how the object is used. Treat both sides as one design and check that the image's orientation feels natural when the card is turned.