i want to add an option to copy img tag of the Thumbnail image and not the img tag of the original image
can somebody help me?

It's the kind of support forum where a lot of friendly generous people volunteer to spend a lot of their free time helping others with opensource software problems and get paid nothing in the process. Theyre also the kind of people who dont appreciate snide comments from users who seem to think we're required to provide them with a service and get uppity if their request out of the hundreds of posts a day goes amiss somewhere.

Now in answer to your question, and I havent tested this so you might have to play with it a little, try opening up index.php and finding:
$FolderPath = substr($Config->CurrentBrowsingDirectory, strlen($Config->CurrentWorkingDirectory)+1,(strlen($Config->CurrentBrowsingDirectory)-strlen($Config->CurrentWorkingDirectory)+1));
$FolderPath = ($FolderPath != "")?$FolderPath."/".$FileName:$FileName;
$EncodedPath = EncodeLinkUrl(FilePath(CurrentWebPath(),$FolderPath));
Then below it, add this:
$ThumbPath = ($FolderPath != "")?$FolderPath."/_thumb.".$FileName:."_thumb.".$FileName;
$EncodedThumbPath = EncodeLinkUrl(FilePath(CurrentWebPath(),$ThumbPath));

Then below that, find:
$Return = "<input type=\"hidden\" name=\"Item$ItemID\" value=\"".$EncodedPath."\" />
and below it add:
<input type=\"hidden\" name=\"ItemThumb$ItemID\" value=\"".$EncodedThumbPath."\" />

Slightly further down, find:
<li class=\"Copy\"><a href=\"Javascript:copy(document.frmLinkContainer.Item$ItemID);\">Copy url</a></li>\r\n";
and below it add:
<li class=\"Copy\"><a href=\"Javascript:copy(document.frmLinkContainer.ItemThumb$ItemID);\">Copy Thumb url</a></li>\r\n";

Would you mind posting the corrections then? That way other people can benefit from Mini's and your work :)