I use KOMA Script which always places images in the top of the page. I don't think that this always looks nicely, but if I change the place of my images it doesn't seem much better. Unfortunately I couldn't find any guidelines concerning the proper placement of images and tables with latex. Could you provide me with some tips for a good layout?
If you can read German you should have a look on page 255 ff in the book "Lesetypografie", ISBN 3-87439-652-5. It shows on several pages how you can use images in a good looking way. I do not know an English book, sorry.
I think that KOMA-Script does (if you let it) a good job with the positions of the images. If you want to choose the position by your own I would suggest to use the position at the beginning of the page. A big image at the end of the page could give you the feeling, that the page is larger than the others, it looks "heavy".
My English is not so good, I hope I made me clear.