Question details:

I can't find a way to reach my Picasa site from Outlook (when go to 'Insert' I get, for example: 'My Documents', 'My Pictures' etc. but there is no link to Picasa anywhere.

Answer:

We didn't find a method to do exactly what you want but we can show you how to insert photos from Picasa albums into Gmail.

You can use a special Gmail Skin which has an Firefox extension lets you customize Gmail interface. It has the feature to "insert images from Picasa web albums into Gmail" (it does not embed pictures, just puts a liks to the images).