Well, "used by another process". Which process? I have seen that there is a thumbnail folder in XMM. Maybe it's a slow conversion due to irfan (set somewhere in options). Then it should happen only once (each pic). When I get such process messages in windows I use the Unlocker 1.9.0 (Freeware) to check which process has catched the file.
In order to achieve what is possible, you have to try the impossible over and over again. Hermann Hesse