Each time you access a file through your web browser, the browser caches (i.e., stores) it. By doing this, the browser doesn't have to newly retrieve files (including any images) from the remote web site each time you click Back or Forward. You should periodically clear the cache to allow your browser to function more efficiently.
Clearing your cache may be required if you experienced an error, such as an invalid password. Your computer may continue to recall this error if it was cached.
Each operating system is a little different than the next. Here is a site that will allow you select your OS and take you step by step through the process of clearing your cache: