There are several types of "Transformations." They are: Secondary Form, Other Side, Additional Reference, and Familiar.
With that in mind, please note that certain characters who require Secondary Forms or Other Side, as well as Familiar tabs, will be marked as Disabled and will be disallowed from participating in regular gameplay until an appropriate reference image is provided.
Secondary Form is used in the case of a Spliced spirit who shapeshifts between their forms. For example, if one kiji's form is a volus with a golden base coat, and the other form is a lien with a nighted base coat, you would need this Secondary Form tab.
Other Side is used if you have a marking such as Skew, Pinto, or Asymmetrical Energy, or a merged Splice that is vastly asymmetrical. Please note here that Asymmetrical Energy and complicated aysmmetrical Splices require an Other Side reference, but it is not required for Skew and Pinto (but it's nice if you provide the reference).
Additional Reference is used if you have extra images you feel the need to display for any number of reasons. For example, sometimes a Spliced character is perfectly merged, but their "alternate form" could simply be a change in energy color. They could also be detailed closeups of certain features (such as eyes), color palettes for characters who may be difficult to normally colorpick from, versions without sheens/shines (Iridescence, Shiny, etc), or versions to show the character with one wing removed to show the other wing or the back.
Familiar is used when the character has a custom Legendary Familiar Slot applied, and optionally for other existing familiars applied. The custom Legendary Familiar Slot must first be discussed with Wolvenhyde, who decides the effects and boosts of the familiar, and then a reference image needs to be created by the owning player and provided for upload. While the character will be disabled if they have a legendary familiar slot without reference, other familiars are optional to have reference. How one acquires the reference image is up to the player, but remember you can't use AI generation or something pulled off Google images.
How to Submit an Alternate Image
Through the relevant character's import page, on the sidebar to the left, you'll need to click on Update Design under the Settings header. From there, proceed to Create Request, then fill out the required information, following the instructions on each page, as if you were going to update the primary image. Be sure to mention in the comments what alternate image you're applying so that we input the correct Transformation field! If we have to track you down to ask if you really meant to submit a crow as an alternate form instead of as a familiar, we'll simply reject the submission and have you redo it.