Can a file attachment image thumbnail appear in a report?

It's an issue with cacheing in your browser settings.  I don't claim to b an expert and it will depend on the Browser you are using, but there will be a setting in their somewhere as to if the browser will chache content. They do that to speed up rendering on the screen, but the result is that it may render old data.

If you are stuck, let me know which browser you are using and I will have a look at my similar settings.