Below you'll find screenshots of most of the custom biomes in the world (or groups of them).
These were taken in a test world, which had a different seed - you wont find these exact locations in the active world.

Alpine - (Clear, Forested, Lakes, Mountains)

Plains - (Meadow, Heath)

Forest - Flying

Forest - Oak Sakura

Forest - Birch Poplar

Forest - Cliffs/Canyons

Taiga - Pinyon

Taiga - Fir

Taiga - Conifer

Taiga - Bald Cypress

Taiga - Webbed

Sometimes appears inside Fir, Conifer and Bald Cypress Forests

Desert - Baobab

Desert - Red Conifer (+ Plateaus)

Desert - Wasteland (+ Plateaus)

Dark Mountains - (Meadow, Forest, Lakes, Mountains)

  

Mixed Mountains - (Forest, Granite Diorite and Clay Mountains)

  

Mesa - White

Mesa - Verde/Lush

   

Ocean - Tropical Archipelago

 

Ocean - Rocky Archipelago

Ocean - Flooded Forest

Ocean - Mushroom Archipelago

Jungle - Cliffs/Canyons

Jungle - Rainforest

 

Jungle - Rainforest Tropical

Jungle - Swampy

Swamp

Swamp - Mangrove

Tundra - (Clear, Wooded, Ice Spikes, Lakes, Mountains, Glaciers)