This may indicate that memory is low. To correct this, simply click the next or previous arrows and then return to the image. Another solution is to zoom on the image by clicking on it once. This allows your computer to use hard disk space to simulate more memory for applications. This works especially well if you have a fast hard drive.
More physical RAM is always the best option however.
This will allow you to increase or decrease the memory used for the browser so it balances with other active programs. This is a typical memory usage scenario.