gimp - I need some help understanding DPI

I have designed a custom deck of cards and I want to send my designs to print. It took me a few weeks to complete my designs using GIMP. I'm new to digital design and made a mistake – I did not set the DPI for the canvas and my designs are set to 96DPI.

What do I need to do to ensure I get high quality prints?

Here's an outline of my project ad what I've done to this point:

• The requirements are 822x1122 at 300dpi


• I did an experiment and the results are confusing to me


• I took a large image from the internet (1280x1008), opened GIMP with a 300DPI canvas and scaled the image to 500x394 and saved it



• I opened GIMP with a 96DPI canvas and scaled the same image to
  500x394 and saved it then copied the image from the 96DPI canvas and pasted it into the 300DPI canvas.

I thought the 96DPI image would shrink on the 300DPI canvas however both images appear exactly the same in size and pixelation when zoomed in.

Are these images the same quality?

I could create new 300DPI canvases and paste my 96DPI designs into it but surely the data is already lost, but images scaled to a 300DPI to begin with do not look any better than my 96DPI images.

Answer

Any PPI requirement only makes sense if it is accompanied by a physical size, and if you're given a pixel size requirement then the PPI is irrelevant. As long as your pixel dimensions are correct then the PPI doesn't matter.

As I previously said here:

PPI is not an inherent property of an image. There is no such thing as a 300PPI image, or a 72PPI image. PPI is just a useful measurement for determining the print size of an image.

Which means PPI is completely irrelevant unless accompanied by physical dimensions. If someone says "Can we have that image in 300PPI?" they need to tell you a physical size in inches or centimetrs or whatever else, otherwise the question makes no sense.

A 100 × 100 pixel image saved at 300 PPI is exactly the same as a 100 × 100 pixel image saved at 72 PPI, or 10 PPI, or 1 PPI. They are even exactly the same if you print them at the same size.

The only times PPI is a useful measurement are...

1. 
   

   You have a physical dimension requirement and you need to know how many pixels you need in your image.

   
   

   Say you need a 6 × 4 inch image at 300 PPI, that allows you to calculate how big in pixels your image needs to be. 6 × 4 (inches) times 300 (PPI) is 1800 × 1200 — and there is your required size in pixels.

   
   


2. 
   
   

   You have an image at a certain size in pixels, and you want to know how big you can print that image.

   
   

   Say you have a 1800 × 1200 pixel image and you want to print it at 300 PPI. 1800 × 1200 (pixels) divided by 300 (PPI) is 6 × 4 — and that is your print size in inches.