If DropDMG itself crashes, the DropDMG Crash Reporter window should automatically come up. Make sure that you enter a valid e-mail address and then click the Send Report button.
If your mail program (or a helper process such as hdiutil) crashes, you’ll need to send in the crash log file manually.
Crash logs are stored in the folder:
/Users/<username>/Library/Logs/DiagnosticReports/
Once you’ve opened the DiagnosticReports folder, find the most most recent crash log file for the program that crashed and add it to your e-mail message as an attachment.