Screenshots disappear

Spark is working great for me with OpenFire but I do have one problem. I am using it for tech support on my lan and have advised people that when they get an error they should take a screen shot using spark and send it to me. If I open the screen shot right away then all is fine. If I try to open it fifteen minutes old then it is not there. If I close the conversation and reopen it then the little icon for the screen shot and everything is gone. Just the text remains. Is there some sort of default after which these things are deleted automatically? I don’t understand why it is there and all works well if I open it within a minute or two but not there if it is several minutes. I would like it to be available at least an hour so it is still there after I get out of a meeting and the like.