images questions

Hi, I would like help to do the following:

To have thumbnail images on my home page next to excerpts of posts, then when the actual post is opened up, the photo would be a regular size within the content, not a thumnail.

I have tried to figure this out, and see it on many blogs, but how is it done????

Thanks so VERY much for any help!
(by the way, I’d need really simple instructions too, I’m not very techie)

Jennifer