Alt attribute should check for image's description and, if not available, the title, like collective.cover does.

[idgserpro]

See https://github.com/collective/collective.cover/blob/68524e2afdbf12fb91c59a284377f394a6cd1e02/src/collective/cover/tiles/basic.py#L127 and https://github.com/collective/collective.cover/blob/e23fed6b55222c54e5f16cb8631a74faddf56616/src/collective/cover/tiles/banner.py#L112 for an example.

Since nitf's tile inherits BasicTile, check if it's possible to use it's alt property: if not, add this method to nitf py tile file.