The Mobile Me Gallery interface doesn't support hot-linking to its images. i.e. there is no explicit URL that will allow you to link to a specific image.

The solution is to download the image from the Mobile Me Gallery, then
upload it to a free hosting service, for example ImageBucket, Flickr etc,
which do allow direct access to images.

We apologize for the inconvenience, but there's no reliable workaround the
Mobile Me Gallery restrictions.

You need to specify a image hosted on a free image hosting site like
Flickr, ImageBucket etc.

An example would be http://farm1.static.flickr.com/15/19176774_a70c1b7351.jpg

We wanted to use images from the photo library initially. However if we
inserted a local image in the email, most email readers like Outlook,
Thunderbird or other web-mail readers do not display it, because this has
been, and still is a big spam issue. Currently only Apple products display
inline-images.

In the end, we compromised by referencing an image hosted on the Internet.

*** Update: this issue has been resolved in latest release ***

Currently Signature only supports the public profile url if you have not created a custom username. Signature has been modified to also accept LinkedIn usernames. The updated application is currently under review with Apple and will be available soon.

Currently we only support the non custom url format ie. http://www.linkedin.com/pub/your-name/number/number/number

The workaround until the next update is either
a) Check if you have the old style link stored somewhere e.g. you emailed a friend or business colleague

*** Update: this issue has been resolved in latest release ***

The new username feature from Facebook was released after Signature was released in the Apple Appstore. Signature has been modified to also accept Facebook usernames. The updated application is currently under review with Apple and will be available soon.