Working with JPEG File in Acrobat 8.0 Pro.

I am new to using Acrobat 8.0 Pro and I am having a problem.

Someone gave me a pdf file and I want to edit it. I am able to edit the text
fine, but the picture is a problem. I want to delete the current picture
and replace it with a different picture. Deleting the current picture is no
problem but I can not seem to insert the new picture properly. I currently
have the picture as a jpeg file but could probably convert it into any other
format if necessary.

What I have tried is opening a new pdf file from the jpeg and I can resize
it the way I want but if I copy it and then paste it into the file I need it
in, the size is large and there does not seem to be any way that I can
resize it (no points at corner of picture) and also the picture itself seems
to be cropped.

I would appreciate any advise. (if this is not the correct forum or
there is another forum, please advise). Thanks in advance