How do I give credit to some image I included in my paper?
First of all,


  permission which could take a while


Is not entirely correct. Most major publishers have an automated "permission request" system that in many cases automatically grants you permission to reuse a figure from a published paper. It takes about 5 minutes and usually you can find it in the journal's home page.


  Now the reviewer asked us to give credit to the original paper. How do I do that?


This isn't too hard. One way, for instance, would be to add to the figure caption: "Redrawn after Smith et al. (2005)". I think it also depends on what the figure is about. Is it a graph with data points that you used? Is it some schematic drawing? If you edit your question to better describe the figure we can give you a better answer.