If you're just using a graphics program, such as Paint Shop Pro or MS Paint
, you can do it, as long as you have drawn scales on both images. I did this before I learned GIS.
Your topo image is your bottom layer, and your cave map is your top layer. 'Simply' use the graphic program's scale tool to match the distance on the map scale to an equivalent distance on your topo image.
The next step is to make the background of your cave map image transparent, so that all you see of that top image is what you've drawn. How you do that depends on the image format and the software you are using. But it can be done.
If you have data that you can enter into Compass, you can export a shapefile to view on Google Earth, Compass' own overlay viewer, or on ESRI's ArcExplorer Online. All free.