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I have a suggestion to allow an image in the signature via webmail that will properly embed the image so that it is not a linked image. I have been experimenting with base64 encoding trying to get this to work, but of course the key is having a proper mime part section to link to so your source can point to it like this src="cid:af4f285892664d06bc6a4d85a1c518a8" and you can not create that mime part in the raw html view because it gets parsed.
So here is my idea - why not create a button while creating the signature that will upload your image and give it a handler that you can place in signature where the image should go. Then the handler will take that image from the location where it is stored on the server and replace it with something like src="/TempResourceHandler.ashx?file=att_187ebd94049946239dc0b416f3a9ca65.png" like it does in the compose window when you embed a picture.
Here is my logic. I normally use a E-mail client, so using webmail is something I am normally going to do when I am not at my own computer. Sure, I can create a signature with a dummy spot for an image, and when composing the E-mail click that spot, click the insert image button, and provide the image from my computer. Yes, that will give me a signature with a properly embedded image. However that creates 2 issues. 1) doing that every time is a pain in the arse and 2) if I am using webmail I am likely to not be at my computer and don't have the image available.
G-mail is already doing this by letting you paste from the clipboard the image into the signature and it ends up properly encoded in your messages. I don't see why the user can not be given the ability to store the image directly on the mail server with an upload button, and have the signature embed it every time using a placeholder.