You can view thumbnail images on shape, action, and image clips in the animation mixer (audio clips are not supported). For example, you can create a thumbnail image of each pose for a character's action sources, then view these images on the corresponding action clips. Viewing thumbnails of the action sources or shape sources (shape keys) is especially useful if the source names are not descriptive or indicative of their contents.
You can use only images that have already exist in the Thumbnails folder of your current project as thumbnails; that is, you cannot use just any image file as a thumbnail.
For general information about thumbnail images, including how to create one, see Thumbnails for Scene Elements [Scene Elements].
Clips with thumbnail images set for them.
To display clips with thumbnails
Choose View
Preferences from the mixer's command bar to open the Animation Mixer Preferences property editor.
On the Clips page, select Show Thumbnails in the Clip Thumbnails area.
The image appears on the clip in the mixer. The size of the image is determined by the Thumbnail Size you can set in the Display preferences (choose FilePreferences and select Display).
If a thumbnail contains a sequence of images, set the Display x Thumbnails For Sequences value to set the number of images to display. The thumbnails are computed at intervals based on the duration and number to display. If there is not enough room to display all the thumbnails, then none are displayed.
If an image clip contains a sequence (.avi, etc.), you can display it by selecting Thumbnails as Sequences. The sequence is displayed using only one thumbnail that is computed from the current frame. Scrubbing the timeline updates the thumbnail image displayed.