How to snap all clips together in timeline at once in Final Cut?

Really? Select all the clips that have gaps in between you want to remove, hit Command G. Open the Timeline Index, type Gap in the search field, select them all, check the timeline to make sure only the ones you want to remove are selected, press Delete.

there is a gap between each photo.

In addition to the method Tom mentioned, please refer to the diagram that explains the timeline magnet function.

●How to fill the gap

Not if they’re connected clips. You could convert them to a secondary storyline and then use the timeline index to delete the gaps.

Too much effort for a simple slideshow type video.

Have you actually tried it? Please tell us specifically what was difficult about each proposal.