Load yahoo mail in one window.
Open a separate browser window so you have your mail in one window and a separate browser in the other side by side. In the empty browser window, upload your pic to imageshack or photobucket.
Drag the image from it's online imageshack or photobucket location and drop it in the message you're creating. You can resize it at that time as well if you need to.
But realize that you're still going to run into the issue that a lot of mail apps won't display embedded pics in an e-mail from a user you don't know.
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