I've just downloaded it and it ran fine, libcfitsio-3.dll is present in imppg-win64.zip. Can you try downloading again, unpacking to a fresh directory and running from there?
On another note: did you copy "imppg.exe" somewhere else (and renamed it "imppg 1.9.exe"?) It has to stay in the same folder as all the other files (DLLs, images etc.). However, it's fine to make a shortcut (link) to imppg.exe (e.g., on the Desktop).
My software: Stackistry — an open-source cross-platform image stacker ImPPG — stack post-processing and animation alignment My images