please help with if condition

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This is to go in postbit_legacy

I am looking for the code so that if they say yes, it will display an image, but the next field would be if they want the preset image or a custom one, where they will enter the URL.

So:

Do you want a banner? Yes / No
Would you like it to be preset / custom? Preset / Custom
If Custom, Enter URL here - URL

How would I do this with an IF condition? Note that the image will be surrounded by <a href=""> to link to another site.

Currently I am using:

PHP Code:

<if condition="$post['field8']==Yes"><a target="_blank" href="http://www.xxxx/$post[field7]"><img src="http://xxxxxx/forum/foundat.png" border="0"></a></if> 


However this is just to show the default image if they want a banner shown