Image Dimensions, and Templates
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.
There is an exception to the 2:3 ratio for temporary covers such as announcement images for new serializations and new anime that can't be reasonably cropped.
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.
Large images will be down-scaled to either 230px width for regular covers, or 460px width for XL covers. So it is fine to upload larger images.
Banners should ideally be 1900x400px for media entries. Uploading slightly smaller or larger images is acceptable, but up-scaling can lower the quality significantly.