save folder

I have a targeted folder for saving.

for example the user chooses C:/destop/renders/image_.jpeg

which that is then stored in a variable such as SaveLocation

SaveLocation = C:/destop/renders/image_.jpeg

Now how can I make it so when the images are rendered it inserts the frame number incrementally so the result is

image_00.jpeg (C:/destop/renders/image_00.jpeg)
image_01.jpeg (C:/destop/renders/image_01.jpeg)
image_02.jpeg (C:/destop/renders/image_02.jpeg)
image_03.jpeg (C:/destop/renders/image_03.jpeg)