Caching and WordPress

Most people will have seen the sidebar graphic with the Curiablog poll average. I update it everytime there is a new poll by replacing the graphic file at curiablog with a new graphic.

By keeping the file name the same, it means other sites (such as Kiwiblog and Policy Net) can have the graphic automatically updated.

But there is a problem. WordPress seems to cache the old graphic and it takes one to two days for the new graphic to appear.

Anyone know of a way to stop this by clearing the cache? Note that curiablog is run on wordpress.org – it is not my own installation.