Apologies for the noob-question, but I want to create a new deck for a homebrew, but I can't figure out to use Strange Eons to put a custom design on the back of the card. I can only select from decks that already exist. Help please

If you are looking to make your own card back without any of the Talisman stuff you are better off just sizing it up in Photoshop and printing a bunch of backs on one sheet. Otherwise, you can choose the "Simple Card Back" in Strange Eons and use any picture you would like but you would still have the Talisman symbol border. Pretty much all of the cards use the Simple Back border so this is the route I have taken...

Agreed.

Adobe Photoshop and Adobe Illustrator are probably the best but Gimp is a good alternative and its free.

Personally I use all three for different effects.

Then get creative .