FCPX: Select all stills in timeline
The fastest way I was able to find by fiddling with the UI is as follows:


Create a new role called "Stills" in the menu bar under Modify > Edit Rolls...

Select everything (both clips and stills) in the main timeline.
Open the index, click the roles tab and uncheck "Video". That way all the stills turn gray and only the videos remain blue (since they contain audio with a role of "Dialogue").

That makes it easy to then manually deselect all the videos by holding command and clicking on them in the timeline.

Now, with only the stills selected, right click the selection and choose Assign Video Roles to assign the newly created "Stills" role.
Finally head back to the clips tab of the index, right click in the column header and make sure Video Roles is displayed. Then type "stills" in the search box. You can then cmd + a the resulting list to repeatedly select only the stills in the timeline.