Search "clickable thumbnails" and you should be all set, but I'll write it out for you.
You need two images: one large, one small saved separately under different filenames. Upload to your account (like imageshack.com or photobucket)
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For the larger image, you need to copy the "Direct Link" and for the smaller one, you need to copy the "IMG" tag. Hope I helped. If you have any other questions PM me.
Good Luck!