The latest version of Gmail Skins Firefox extension, that lets you customize Gmail's interface, has a very useful feature: insert images from Picasa web albums into Gmail. The dialog that lets you choose the picture is very nice and fast (it was made by Google for another purpose). Another thing to know is that Gmail Skins doesn't embed pictures from Picasa web albums, it just puts links to the images on the web.
Another interesting feature is that you can insert gadgets from Google Personalized Homepage. The extension shows only the first gadgets from your Personalized Homepage (I can see only the the first two), but in a future version you'll be able to choose the gadgets. Google Reader and Google Calendar gadgets are probably the best choices.
Picasa Web Albums review
Change Gmail appearance
Check multiple Gmail accounts
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