I just need to do something with the icons of Albums where there are only videos.

What do you mean with icons? THe thumbs? You need to upload sidecar images for the videos.https://www.zenphoto.org/news/problems-with-albums-and-images/#------thumbnails-for-videos-or-other-non-standard-

If is film only in a folder - no thumb,
more - in folder images in theme zpBootstrap, thumbs are as seen on the website (including mp4Default.png), but replacing them does not help.
Of course, when I add any image to the folder, it will be this image on the album cover.

Did you read https://www.zenphoto.org/news/problems-with-albums-and-images/#------thumbnails-for-videos-or-other-non-standard- as @acrylian suggested ?

An album does not have a thumbnail by its own. The album thumb is taken from one of the images inside the album. Which image to use as album thumb can be set on the album-edit page on the backend.

For video you have to create a thumbnail(called "sidecar image") yourself and pace it in the same album as the video.
This sidecar image can be anything like .jpg, .png, etc and it has to have the same name as the video!

For example, if you have a video named koncert.mp4, create a sidecar image named koncert.jpg en place it in the same album as your video..

So, in you album you now have koncert.mp4 AND koncert.jpg.