santosh,
Generally you will see these types of errors when an image was not properly uploaded to the system. For example, if there is a permission problem with the mediagallery/mediaobjects/ directory, it could cause either the thumbnail or display image to not be created properly during upload.
Look in the Geeklog error.log file for any clues and also look in your web server's error log if you have access.
Also, can you let me know which version of Media Gallery you are running, there is a stable (v1.2.x) and development (v1.3.x) branch, so it helps to know specifically the version.
Thanks!
Mark