This is likely specific to iOS devices but i dont think switching to "Imagemagick" is going to solve that problem.. I use Imagemagick on my own forums and this is still an ongoing problem on my sites..
I think in my case it might be my gallery plugin.. (not yours) I dont recall the problem being thread related.. I do recall inquiring about this very issue in the past and the consensus was likely a server misconfiguration? its doesnt happen enough that its really a problem. But i digress, this isnt my thread..