Hi - thought I would write a quick note in case other people were having the same error. I'm not sure if this is a problem with just disk images or if it is true for normal (physical disk) partitions as well.
The error is that when you try to resize a partition to make it larger, you get the following message: "MediaKit reports partition (map) too small." Other posts online suggest that this can be due to certain types of other partitions on disks (such as NTFS) that OS X may have trouble resizing. In my case, however, the only partition on the disk image was an HFS+ (Mac OS X Extended) partition.
What ended up being the problem was that it was a GUID partition scheme. While I could increase the overall size of the disk image, I was not able to increase the size of the partition (leaving a bunch of empty, unpartitioned space at the end of the disk image). The solution I found was to reformat the disk image (WARNING: this WILL erase any data on the disk image) and partition it using an "Apple Partition Map" instead of the GUID Partition Map. Upon switching that, I was able to resize the HFS+ (Mac OS X Extended) partition.
This is kind of an odd error, because GUID is the partitioning scheme used to boot Intel Macs.
Hopefully this helps other people who may be having the same problem. To check what partitioning format your disk image uses, mount it, then open Disk Utility. Click the disk (not the partition), and look at the bottom of the window for "Partition Map Scheme." If it says GUID, and you're getting the error above when trying to resize partitions, that may just be the problem.