Think about this for a moment. You load in a single frame and what should appear when viewing it as video? One frame out of 30 in the first second. As a matter of fact one progressive frame in an interlaced 60 frames per second.
Final Cut Pro knows well enough that it will not work in a video context, so it needs to render additional frames of the same image to turn it into video.
As it stands and hopefully will not be much longer, FCP deals with one codec on the active timeline per project. If you introduce ANY other format besides the one that matches the timeline, you will need to render EVERYTHING not matching the codec selected for this project. And if you so much as twitch on the timeline, RENDER AGAIN.
So, to answer your question - the very best way to "edit" stills, is to find a way to turn those stills into video at the same format(codec) as the timeline. One way might be to open another project set to the timeline codec of the finished project and do only the stills to video there. Render that work out, once and then import that work onto the final timeline and there will be no rendering, because of those stills.
There are definitely other ways, but that one stays in FCP.