Hi sbreckan ,
After you define the expression, you will only see the symbols corresponding to the current scale. In case you have a single symbol, this is not a problem, since you can use the "Other" option to define the alternative symbol. As shown below when the scale is 50K or more, the symbol is a simple square:
When we zoom in one step (view scale below 50K), the symbols change to be a larger house. You can define any symbol including an image using an URL: