Pictures frames allow you to upload any screenshots you've taken from the PDA gallery. You can also put any image you want onto your gallery by putting a file into the screenshots folder of the save, and then uploading the image to the picture frame.

This folder can be found at: Steam > steamapps > common > Subnautica > Subnautica_Data > SNAppData > SavedGames > slot**** (find your save file here. They are in order of when you created them) > screenshots

The folder should also have any other images that you have in your gallery. Note that this will only show up if you have taken screenshots with F11 in the PDA. These are not the same as steam screenshots; the steam screenshots are seperate. Now just put all the images you want into this folder. I recommend using JPG files.

You can now upload custom images from your gallery to picture frames.

For Subnautica in Epic Games: %USERPROFILE%\AppData\LocalLow\Unknown Worlds\Subnautica\Subnautica\SavedGames\slot(xxxx)\screenshot