How do I remove missing Assets from Build / Import (Reimport All) ? 5.2.2.2f1

Hey folks,

i got a problem with some Assets in Unity. In my unexperienced first steps I tryed to Drag&Drop some Images directly from the windows-folder to the unity-folder of the unity-editor(v. 5.2.2.2f1). Badly it wasn't copying the stuff to the folder but anyways something has happened inside of unity.

Later on I had to reimport all stuff because of an unknown UnityEngine.UI Lib bug, which always crys out errors after the first save action of the started unity program. No problem, because afterwards it's solved, as long as unity itself is running. After the "Reimport All" unity complains about "Could not create texture from Assets/xxxx.bmp : File could not be read". Any way to get rid of this error - fixing this with a build fix? Is there a xml-build file, which i can manipulate?

Greetings and sorry for my horrible english.

Stev Oh

P.S. Badly i m not able to choose 5.2.2.2f1 or sth useful in the topic field, so i used 5.3. Is this a known bug - is there a fix in 5.3? Ty for every useful answer ;)