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this is a great solution: OIOpublisher Ad Manager - Control Your Ad Space
but it's not free.

There are several ways to achieve this...

1. With a regular text widget you can place ANY kind of banner using:
<a href="the link url"><img src="the banner image url"></a>
2. modifying the...

First thing to try is to re-save your permalinks. Literally just click the save button and try again.

If this doesn't work, you may need to check the actual permalinks structure you are using, and...