Excel made it remarkably easy to change from GIF to PNG formats (just changing a parameter). And the image quality is pretty much the same, so that worked well. Thanks!

I like your first approach, since the chart data is coming from a query returning multiple records. If I read what you are suggesting correctly, I don't have to worry about time-stamps at all, just check the hash values. That would allow much longer time-to-refresh values for the files.