Hi Jan,
I might suggest using Photoshop or Fireworks if you have them and are comfortable giving them a try - and yes, just create the image you want to use, add the textual content and save the image. Ideally though, you could, create something using HTML - if you're able.

But it sounds like that might not be the best option for you - have you looked into what CC offers in the way of ready but editable postcard templates? Most of these email programs have a nice library of templates you can start with and add to as needed and although I haven't been in CC for ages I seem to remember they had some fairly nice designs to work with - check it out and post back if you don't find anything.