Search results may include any images that are not prohibited from indexing in robots.txt or that are published on pages that robots.txt does not prohibit from indexing.
Texts describing the image are important for image search. The search engine can assume what's displayed on an image by analyzing texts. Yandex finds images for users' queries precisely from the texts that correspond to images. That's why images that don't have content descriptions are not included in search and cannot be found for queries.
Yandex only indexes images with standard graphical formats (JPEG, GIF, and PNG). Background images and pictures shown using script are not indexed.