Each year I ask myself, "Do I really have to take more flower pictures?" I hold off until the beauty just cries out to be captured. We bought this delphinium plant yesterday. So I don't forget how I did it: The frame was created by making a selection, inverting it and then using gaussian blur. I then re-inverted the selection and did an edit/stroke with one pixel of white. (Yes the frame is a little too wide on one side...A little brainwork and I came up with the following, which will insure even borders: Easiest would probably be to scale down a copy of the image and then paste it. That way Photoshop automatically centers it.
Here's how in Photoshop:

Open your image.

Go to Image/Duplicate. On duplicate image: Select All/Edit/Transform/Scale/Enter New Size in Height Width in options. Click on Checkmark to accept. Edit/Copy.
Click on Original Image and Edit/Paste.

Go to Filter/Blur/Gaussian blur and slide to your liking.

Click on duplicate layer and use Magic Wand to select blurred area.

Go to Select/Inverse

Set your background color to the color you want for your inside border. Go to Edit/Stroke. Select number of pixels (1 or 2). Select "Inside". Click Okay.