those of you with tumblrs - please help me with posting pictures

Somehow I have failed tumblr 101. Every time I try to post an image, it either posts it very tiny, so that you have to click on it to see it, or it doesn’t even post the image at all — just a grey box that you click on to see it. I have seen many, many people post images on Tumblr that show up as their normal size - I know the width limit is 500px, and I know the filesize is small too.

I’ve tried it by linking my to a URL at my own hosting site; I’ve tried it by uploading an image to tumblr; I’ve even gone into the page source for people who have posted images that show up normally and *copied out the source code and replaced the img src URL with my own* and created a post that way, and it STILL doesn’t work. Can someone help me out here?