Well I worked out a solution. I don't know if it is the best all around solution, but it works for my needs.
In the scriptBlock where I register my toggle function, I added the following:
scriptString +=
"var minImage = new Image(); \n";
scriptString += "minImage.src = \"../App_Themes/" + Page.Theme + "/images/Minimize.gif\"; \n";
scriptString += "var maxImage = new Image(); \n";
scriptString += "maxImage.src = \"../App_Themes/" + Page.Theme + "/images/Restore.gif\"; \n";
As long as each theme has 2 files with these names, I'm golden.
-Gary