there are like a million different ways to do this.. some call for adding codes in your template as well as adding an html gadget, only because there are endless options on how you want your little guy to pop up. so this is the easiest way, with the least amount of effort haha. you're welcome...
copy & paste this code below inside your html gadget.
replace the "IMAGE DIRECT LINK" with yours. (to get your image direct link, upload your image to photobucket and you can retrieve it there. if you need more help with this, shoot me an email)
locate your gadget somewhere towards the bottom of your layout to just have it out of the way. but honestly, it really doesn't matter where it's at.
that's it! view your blog and once you start scrolling down, your image should pop up at the bottom of your scroll bar.
if you need any help or have questions, let me know!