Unless Skype has a secure address for you to pull the image from you can't, but if you search the forum I believe you can disable the sidebox from showing on secure pages

I noticed as well that you need to put your actual Skype name/address (?) in some places in the code:

<a href="skype:YOUR_NAME_HERE?call"><img src="http://mystatus.skype.com/balloon/YOUR_NAME_HERE" style="border: none;" width="140" height="56" alt="My status" /></a>