I use Spriter with Construct quite a bit. If the image you are adding is exactly the same size and the key points in the image are in the same location, you could be fine swapping it in the construct files. Given the state of the Spriter plugin, it would be easier to do it this way. You will have to find ALL the different places the image you are replacing is in the construct file and make sure the new file is named the correctly but it can work.
If the new image is not exactly the same size with all key points in the same positions, you definitely want to do it in Spriter.
If you are running the beta version of Construct and have the version of the plugin that is built for the beta, I would say you would be better of doing the change in Spriter. The beta version of the plugin is MUCH easier to load edited versions of the project.