TL/DR Copy and paste the following © in to your pinta text box ©

I am no expert with Pinta. I do have Linux Mint but not using that at present, but could no doubt start up and try Mint and install Pinta on it if necessary to try to give a correct answer.

Meanwhile have tried and succeeded in using a copyright symbol in a Pinta image. This is how I did it:

• I did not see any method of directly selecting a Copyright symbol: ©

1. I used Libre Ofiice Writer and added a © symbol and copied that by selecting and using keyboard shortcut Ctrl C to copy
2. I then opened a pinta image and a text box that I was typing in to.
   using Ctrl V I pasted the symbol in to the text box.

• Presumably you will need to select a Pinta font that includes © but that will be most fonts
• Whilst reading this you may of course simply copy and paste the © from this answer

I hope my answer helps you juniperina, or other Pinta users.