Search results may include any images that the site does not prohibit from indexing in its robots.txt file, as well as images posted on pages that robots.txt does not prohibit from indexing.
Texts describing the image are important for image searches. The search engine can assume what's displayed on an image by analyzing its text. Yandex returns images whose text relates to the user's search term. That's why images that don't have content descriptions are not included in the search and cannot be returned in response to queries.
Yandex only indexes images in standard graphical formats (JPEG, GIF, and PNG). Background images and pictures that expand using a script are not indexed.