Create calculation and add the condition based instructions.
Drag the calculation into label and select- selected option in label.
if your condition satisfied then the label will show.
if count([Customer Name])>1 then "This customer purchased more then one time" else "" end
but it's not quite what I'm looking for (although very useful to know!)
I want to put a picture on first highlighting how a user can click on filters, click on charts to change views, etc, and then for it to disappear as soon as they start to work with the report.
Although it sounds like a URL action that triggers on click may be the best way to do this, I was not able to do it. I think you would want to set up two dashboards, an interactive one, and a static picture (with annotation) and the action would force the user to the real dashboard
A second, but easier method would be add an annotation parameter. You can also use the annotations with an on/off selector to provide some initial instructions, but the user will have to select the box to turn them off. See my workbook here on : | Tableau Public
Does your requirements allow a parameter to be in use?