Hmm. I assume you're posting them with some automatic option? It says, "When uploading from file or from ScrapBook, the image will display as 320 x 240 (medium) and will link to the image in ScrapBook" which might be your problem.

I always edit my posts' HTML manually, so I just post: <img src="url"> If I want the image to have a black border, I use: <img src="url" style="border: black solid 1px;">