You have to resize the image not compress it. First off I will recommend GraphicConverter. Open your image in that program and you should probably resize the image and go to PictureMenu>Size>Scale, and reduce the size to around 75% (or less if the picture is really big). Then do a Save As... and choose your file format (JPEG), and when you hit save it will bring up another window that allows you to change the quality of the image and it should tell you what the file size is going to be.
This is kind of a crash course in file resizing so if you have more questions let me know. Or search the forums for this question, it's rather common.