A download items for the Tamagotchi iD L can be found here: https://tama-zone.com/index.php?showtopic=24523 (go to the last post in that topic, then work backwards until you find a collection .zip file, then work forwards) and here: https://www.gotchiworld.de/ (hover your mouse on "Download iD-Series", then click one of the "iD L" categories from the menu that'll appear.) If you acquire the download item as a .zip, extract them to .jpg. Don't attempt to edit the download .jpg files using a paint or photo-editing software. Doing so may corrupt or delete the Tamagotchi data in the file, making them unusable as a download file.
Once you have the download items on your PC, you need to beam them (one-by-one) to your Tamagotchi iD L. To do so, you need a compatible IR device. The most reliable method is to use an older model cell phone (the phone doesn't need to be on an active plan to do this.) A list of devices known to work can be found here: https://tama-zone.com/index.php?showtopic=22536