Under your body part properties, just uncheck the box that says "allow part to break across page boundaries". I would also turn off auto-sizing on the container field if you can.
So you have text and other fields that you want to appear above that might take up most of the page, and want the container image to slide up, but only when there is space? Mike B is on the right track, but if the other content is longer than a page, it may not work.
You can put the container image in a sub-summary part and sort on a constant field. For example, you have a field that has a constant value in it, like "1", that you can sort on and it won't make a difference, but will cause the subsummary part to show up. On the subsummary part, uncheck the box to allow break across page boundaries. Then the image will slide up and print on the page only when there is room, otherwise it will print on the next page.
This worked for a colleague that ran into this for a report. Hope it helps.