Set background image in a post

Hi,
I'm new to the blog world, but I must say I love this theme. I've got quite a bit of web experience, but need some help on the image thing.

I'm trying to set a background image in my post. I will likely want different images in each post to make them artsy. I'm posting in a table, and then adding the table code to the post with the image set as the table bg. Shows up great in my preview, but live on the site it shows up as a grey box that 'flashes' as you mouse over it.

Any suggestions? All help appreciated.
nt