All images (except banners) should ideally be close to a portrait 2:3 ratio.
Volume covers for manga and light novels - and many key illustrations for anime - aren't an exact 2:3 ratio, however, they should be uploaded untouched.
If you have to crop or create new images, for things such as cutting out parts of larger images for better staff and character portraits, then 230x345px is the recommended minimum, or at least 460x690px to support the XL cover.
Banners should ideally be 1900x400px for media entries. Uploading slightly smaller or larger images is acceptable, but up-scaling can lower the quality significantly.