Preview files should locate in a folder
Atom Preview Assets (near the package file .ATOM and utility files) , duplicating the hierarchy from your package structure.
For a preview of one element you need two types of files:
- Static picture of the element (format
- Animated review of the element (format
The titles (PNG & GIF) of one element have to be the same.
Possible proportion and size (pixels) for elements:
Set up as a default for all the elements
There is no need to create a preview file for audio files and similar elements (when there's no need in displaying of the element) - the extension will automatically create a review template.
The aspect ratio is default 16:9
DEFAULT, but you may set any variant from listed above for certain groups (see more details here).
Regardless of the suggested size, you can use any other (to the higher side) - the extension will zoom it up or scale it down in a correct way.
The important thing is to preserve proportion for types of elements.
P.S: If it's possible, consider the size of GIF animation, and if the size is too big (more than 2~5 Mb) use compressors (for ex. ezgif.com) to reduce them with no loss in quality.
More details on preview files location see in chapter package structure.