Here is a tutorial to help you make things with borders in PowerPoint. I like PowerPoint better because you can move things around where you want them!
1. To start add a shape- I normally insert a square.
2. Then right click on it and click format shape.
3. Fill- You want to select fill and "picture or texture fill" then choose the photo from your computer that you want to use as your background. My theme is black with white polka dots so that's what I use.
4. In that same box (format shape) you can choose the line color and style.
5. Then add another shape and leave the background white (make sure to match your line color and style here too).
6. Then add a text box on top of everything and type what you want!
Ta-da! That should make it so you are able to make your own custom posters with cute borders. Please let me know if you have any questions!