You didn't explain how you are storing the images nor how you distinguish Character 1 images from those of Character 2, so I will assume you are using either foldernames or filenames to do this.
... you also disn't include an example of the code you're using to display the image so I will assume that it is something like the following.
If the above assumptions are correct then you could use a custom widget/macro combined with a known story variable to achieve the desired effect.
1. Initialise the story variable used to track which Character's images to display by default, this ideally should be done within your project's StoryInit special passage.
<<set $charPath to "char1">>
2. Create a custom widget that knows how to generate an img element based on the name of an image plus the current value of the $charPath story variable. The following code needs to be places within a widget tagged special passage.
\<<set _image to $charPath + "/" + $args>>
3. Use the new widget to display an image for the current Character.
4. To display the images for a different Character simple change the current value of the $charPath story variable.
<<set $charPath to "char2">>