Answer: There is no way to change the DPI, the DPI of an exported image is always 72.

Workaround

There is however a workaround enabling you to create a high resolution image of the DPI you require. I will walk through the different steps and using Photoshop as graphic tool.

There is a handy calculator that will help you to calculate the required pixels depending on millimeter and DPI.

Example

If you want to have an image in A4(210x297mm) format with a DPI of 300 at the end, you need an image that has a dimension of 2480×3508 in pixels.

Step 1

I created a simple image with photoshop representing my business card. I want to use a 85x55mm image with a DPI of 300 and I would like to have the same image parameters again after exporting it in the order details.
With the calculator I need pixel dimensions of 1004×650 for that purpose. In Photoshop it tells me the same.

Step 2

Now I am going to create a new Fancy Product under Fancy Product Designer → Fancy Products and set the width and height for the Product Designer to the same dimension I am using for my image in Photoshop.

Step 3

After the customer makes an order, we can receive the ordered product in the order viewer and export it as image or PDF. The image will always have a DPI of 72. When we open the exported image in Photoshop and open the “Image Size” window, which can be found in the “Image” menu, we will also see that.

Now we only need to adjust the parameters to our desired DPI and size.

That’s the only way to create an image with your desired DPI.