hello
I am a little intrigued by this practice: a person buys an image on alamy, then cancels his purchase and finally bought again the next day at the same price but not with the same definition in pixel !

Happens all the time.  It was either a purchase mistake, or the buyer found a cheaper way to license the image.  My main concern is that if it is the former case, the buyer already have the downloaded image.  If the buyer gets caught using it, what is the penalty?  The worst case he just pay for the usage cost.  Let say he pays of 1 year of usage and the image continues to be used 5 years later, who is keeping track of it?  The policy of letting buyers refunding images, won't work.  The one suffering at the end are the contributors.