GoPro Studio will import ALL photos from a folder and display them as a timelapse sequence if you select just one photo from a group.
If you wish to insert one photo into the timeline there are a few things to do.
Any photo you want in the final video will need to be put into a folder on it's own with no other files.This is the only way to import one single still image.You will need to repeat this for each individual photo you want to use in a project.
When you import that photo into GoPro Studio (STEP 1 View & Trim) look in Advanced Settings and choose the length of time you want the photo to show in the video.
The choices are 2 seconds or 5 seconds.
You can import the photos as described from individual folders,apply desired settings and then convert a number of photos at the same time.
In STEP 2 EDIT,if you want a photo to show for more time put the same photo into the timeline next to the first and the photo will show for twice the time.The photo in the media list can be dragged to the timeline repeatedly as many times as you like.
So this means if you have a project that you want video and still photos in you must go through the import and convert stages twice,once for video and once again for photos.
I hope this sends you in the desired direction.Enjoy.