Thank you for downloading my resource pack! It means a lot! This is only my second pack and I made it to be completely customizable.
To change this pack, please keep these things in mind:

Out of the box, this resource pack changes the Item Frame into a Shadow Box made of Spruce wood and backed with Red Wool.

The textures on the Shadow Box are pulled straight from the Minecraft textures library. This means that, if another resource pack changes the look of Spruce Wood (or whatever material you decide to use), then that same texture will reflect on the Shadow Box.

Understanding that, this is how you can change the look of this resource pack!

Follow the assets folder all the way to assets>minecraft>models>block and open up item_frame.JSON. This gives you a bunch of numbers which represent the different polygons that make up the model.

At the very top, you'll see a "textures" field, followed by "glass", "backing", "frame" and "particle". These are identifiers used to pull textures from the game.

Put simple, if you put "backing": "block/blackstone", then instead of the Shadow Box being backed by red wool it'll be backed with blackstone!