In Atom, support for videos added to a project is dependent upon the video support built into jAlbum. For videos to work properly in an album, video support must be enabled in jAlbum under Tools (jAlbum in macOS), Preferences, Advanced.
If the processed video dimensions are larger than the image bounds chosen under Album » Settings » Images » Image bounds » Images, the display will be reduced to fit within those bounds and within the browser window, whichever is smaller. If you choose Link to originals for the project, or Use original for one video, the video will be shown in its native size, reduced as necessary to fit within the browser window. The actual video file in the album will not be adjusted - only the display window is scaled down as necessary.
With video support enabled in jAlbum, the core program automatically provides a thumbnail image for the video. In image editing mode, a slider is available to choose a video frame other than the one automatically selected. This can be especially helpful if the video starts with several seconds of a black screen - the automatically-selected frame might be black in that case.
It is possible to provide your own representing image for the video, which could be an image that doesn't even appear in the video. Choose a full-sized image (a regular JPG file, and not just a small thumbnail), change its extension to .thm, and give it the same name as your video. For example, for a video called mydog.mp4, the image file should be named mydog.thm. This file should then be added to the project at the same time the video is added to the project. This image will then be used both for the thumbnail and for the video frame while the video is loading and after it has finished playing.
Your web host must provide video files with the correct header information. The MIME type must be video/mp4 - if an MP4 file is delivered by your web host with any other MIME type, playback will fail in some browsers. If you find that a video in your album plays properly when you preview the album locally, but displays only an error message when viewed from your web host, an incorrect MIME type is the most likely cause. Ideally, your web host should correct the server configuration, but if you can change your own .htaccess file on the server, adding this line will correct the problem:
AddType video/mp4 .mp4
In addition, video files must not be compressed by the server using something like gzip, for example.