When Piwik is showing a red cross image, or a broken image icon, it means that tracking visits is failing on your Piwik installation. It is not expected and should be investigated to fix the root of the problem as soon as possible (in the meantime, your visits are most likely not recorded).

To see where the error comes from, open your website in Firefox, right click on the broken image icon, and click on “View Image”. This should redirect you to the URL “yourwebsite.com/piwik/piwik.php?…” and the error message should be showing.

If the error message mentions MySQL, the error might come from a broken mysql instance or tables. Try to “repair” your MySQL tables (using phpMyAdmin for example).
If the error mentions “failed to open stream: No such file or directory” this might be caused by missing Piwik files. Make sure all Piwik files are properly uploaded.
For other errors, try to search on the search engine on Piwik.org.
You can also try the Piwik support forums for more help.