To upload a photo you must be its author, or have an author rights authorization which allows you to upload it.
Also, to upload a photo on which people appear (if you do model photography, that's probably the case!), you need an image rights authorization from each people appearing in your photos (commonly now as a "model release")
This is a legal requisite in Spain (other countries have similar laws) and is managed by the Intellectual Property Law (Ley de Propiedad Intelectual) in the case of author rights, and by the intimacy and own image rights (Derecho a la intimidad y la propia imagen) in the case of people appearing in your photos.
Litmind can't verify all the photos to check for their publishing permissions, so this responsibility is held by the user.
Having the author rights and/or image rights is almost completely required when you do any job, specially if it's a model shooting (TFP is no exception!). We provide you with a bunch of reference documents for image rights and author rights releases, you can find them here.