Signature - missing plugin

Hi there,
I have several email addresses and each has its own signature, some with jpg/logo etc and others simply with text.

Since upgrading to Snow Leopard I seem to be losing my jpg images in the signature blocks.

For my main email, it appears as though it is there when I create a new message, but I have been told that nothing shows up for the receiver. (I am now wondering if that is simply cached).

I have just tried to edit one of my other signatures and notice that all my signatures now show "Missing Plugin" where the jpg should be.

I have tried re-dragging the jpg file to the signature, but still Missing Plug in.

Any suggestions please: I like to keep my signatures professional and a missing plug in does not help

Thanks!