Hi Lewis -
When you upload a file as an attachment to a task, the file is stored in Google Drive, and only in Google Drive. GQueues does NOT have a separate copy of the file. So if you delete the file in Google Drive, then it is actually gone and you will no longer be able to access the attachment from the task. If you click an attachment link on a task, and the file has been manually deleted from Google Drive, then you will get a "file not found" error page from Google Drive.
There are a couple of things to understand about Google Drive though, which may help explain the behavior you have noticed. When you "delete" a file in Google Drive, it's not actually deleted. It's just moved to the "Trash". So it's still available, and will still be accessible when you click the attachment on a GQueues task. The file is only truly deleted when you empty the trash in Google Drive. At this point, the file will no longer be accessible when you click the attachment on a GQueues task.
Also, GQueues does cache images on the server for a short period of time for performance reasons, so a deleted image may still be visible for a little while in GQueues after it's deleted in Google Drive.
There is no "storage limit" for attachments in GQueues because it is completely dependent on the amount of storage space you have in Google Drive. Your GQueues attachments are stored in Drive, so they are subject to the normal limits on your Drive account.
I hope this helps.
-Cameron