While it doesn't have to be a flower, you'll want to choose a vertical image to fit a vertical card. Think buildings in a cityscape, the tower of a castle or a tree with spring buds.
Then you'll do two sets of double mats. The back two will be a solid color of card stock and a subtle pattern that won't detract from the image. The top two will frame the image on white with a coordinating mat behind it.
Add a horizontal sentiment across the whole card and you'll have a beautiful card ready for brightening someone's day.
I chose a light purple striped patterned paper, the same color as the flower, and matted it on a deep purple card stock.
The "thank you" was printed from the computer on white card stock and matted on a coordinating purple card stock. Then to tie everything together, I matched the stripes of the patterned paper for a strip across the entire card.
This card, along with many other paper crafts and gifts, can be found in my Etsy shop. You'll find lots of great ideas and maybe a gift or two!