Editing Calendars in 7.1 and Thumbnails

I wonder if someone can help me? I am adding a Calendar to a web site using 7.1. I am populating the calendar from an Events page. On the Events I am using I add a thumbnail picture to display in the calendar entry.. This works well until I try and edit the calendar, if I try to add spacer blocks and resize the calendar I can do it OK, however as as soon as I save it I lose the thumbnail image on the Calendar. Has any one else had experience of this situation? Or is there a way to successfully edit a calendar in 7.1 without losing the thumbnail. Appreciate any help and guidance.
I am wondering if it is anything to do with the size of the thumbnail after I have edited? Just a thought.

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