I went through the same problem at the weekend and, despite the fact that this thread appears to be non-active, I'm posting a response so that there is an answer for others having the same issue.
As best as I can tell, in OS X 10.5.4 there is no way to resize a Disk Image file and its associated Partitions in a single command without losing data in the image, although there is an option that implies this should be possible. However, the following instructions will enable you to resize a Disk Image and the Partition(s) that it contains to provide more space:
- Open your Disk Image file in Disk Utility and ensure that any Partitions mounted have been Ejected.
- With the Disk Image selected, click the Resize Image button on the toolbar.
- In the Resize sheet that appears you will find that you can make the Disk Image smaller but not larger. Select the arrow button below the slider tool to display more options. Select either "Resize Partition and Image" or "Resize Image Only" (both seem to do the same thing). You should now be able to use the slider tool to select a size bigger than the current size. Use the drop-down menu to select between different file size metrics (e.g. GB) to make your selection easier.
- Select the Resize button to begin the resizing process. If you have specified a large size then this process may take a while (minutes) and the Disk Utility will become unresponsive. Go grab a drink or something.
- Once the Resizing process has completed the Disk Utility should show the new size for your Disk Image. However, the size of your Partition in the Disk Image file will not have changed so there will be no additional free space. The Partition itself must now be resized.
- Ensure that the Partition in your Disk Image file that you want to make bigger is mounted (double-click the Disk Image file in the Disk Utility or Finder if it is not).
- Once the Partition is mounted, select the Disk Image file itself and select Partition tab from the main area of the window.
- Using the mouse, click and drag the bottom-right corner of the Partition that you want to make bigger so that it fills the available grey space. Alternatively, you can type the desired sized into the text field, or add additional Partitions into the free space using the Plus button. Feel free to go nuts.
- Once you have specified how you want to use your additional free space, select Apply to non-destructively apply your choices. This should only take a few moments to complete.
I'm pretty certain that the "Resize Partition and Image" option should resize the existing partition(s) so that it fills the new image file but this does not happen. Hopefully this apparent bug will be fixed in later versions of Disk Utility but in the meantime manually extending your current partition seems to do the job. It was this last step that had me scratching my head for sometime at the weekend and I hope these instructions help others with the same problem.