The resolution of the cards are high, just to make better printouts. You can either print a backside of the card or just write a message with a pen.
I use Epson color printer. Every printer has it's own settings, so I can't tell you how to make the settings on your printer. But I do reccomend that you make some tests on plain paper first. Check your settings and when it looks good, print your card. Always have a check at Print Preview, before you print on photo paper. If you print the back of the card, notice how you insert the picture, to make the address lines look right.
This is the way I print my cards.
I use Epson DURABrite photo paper, 10x15 (4x6 in). They come in a package of 50.
File. Page setup: Post Card, Auto Select, Landscape, Fit to page. Properties: Paper Post Card, Card stock.
Print. Properties: Best Quality, Paper: Premium Photo Glossy Paper. Size: 10x15, Photo. Landscape.