Here is a collection of all extracted chat background/patterns came with Telegram in their original resolution, extracted from Telegram for Android 5.14.0. In Telegram, they are also sometimes called Wallpapers.
This consists of 39 photographs that are there since no long after first Telegram release, 7 patterns in PNG added sometime after that, and 33 patterns in SVG (vector) added recently in 2019. All patterns are monochrome — black drawings and transparent background.
39 Photographs
7 PNG Patterns
Patterns shown here are painted white if you are using dark mode, the original files are untouched.
33 SVG Patterns
Patterns shown here are painted white if you are using dark mode, the original files are untouched.
I had to say that Telegram has put in a lot of effort to design those patterns, subtle but still full of cultural elements. There are things about hobbies, life, festivals, animals, places, maths & science, food, fantasies, nature, space, fictions, and most importantly, Telegram itself. I am very impressed by how the team has made the decision and the efforts to those tiny details that contributes to a better user experience.