This is basically IMGXIS for TweetDeck - its lets you get a close look at images, GIFs, and videos without having to open them in a new tab and/or having to save a local copy.

Effects:

• Clicking an image opens a pan-able, zoom-able view of its full-size version.
• Clicking an avatar in a profile popup: ditto
• Clicking a round white button in a profile popup: ditto, for profile background
• Clicking a GIF: ditto (might look slightly odd because GIFs are videos, but you can play/pause/rewind/etc)
• Clicking a video preview: opens a large embed. This is because I don't see a good way of sampling video dimensions out of iframe, especially for external videos. Videos inside single-tweet views will still play inline.

For each case, you can press Esc to close the thing or click on the underlay to do so (like with profile popups, really).