Drag the new images over the old ones on the timeline. If you make a mistake press <ctrl-Z> to backtrack, step-by-step.
Or delete the old images, place the scrubber (red cursor) where you want a new one, right-click it in the BIN (file box) and Place on Sequence at cursor.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. Then you will be protected, as long as source files haven't been moved or deleted. And older VPJs will be available.
As for audio, lock the track by right-clicking in the area to the left. Then it won't be affected by your still image changes.