You can go to google or other website that have images to get a image for your signature.
Copy the link of the image ( But it have to be the right size or it won't work )
Click this after you copy the link click this link
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Then click something that called insert image then past the image on the white box and hit preview image if it show up. If it does click save and then your done.
EDIT: Oh so your talking about a moving image. That call a gif, ok go to this website there are lot of gif image there.
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Click the image once and right click it to click "Save Image As" to put it on your computer hardware
Then go to this website
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and click computer
Find the gif image your saved on your computer hardware and pick it.
After that click start upload and it will be loading. Once it done copy the link below that say Direct Link (email & IM)
One you done copying the link that under Direct Link (email & IM) go to this link
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click a button called Insert Image, Past the link you copy from imfur and click preview to see if it move or appear on the top. If it does that click save and your done. You have a gif image on your signature.