This is in the setting of inline preview. The commands should be "dvipng". It seems to be a bug of Qt since my Qt Creator has some similar bugs like this.
Does it also happen with Qt 5? There's a test build for Qt 5 here https://sourceforge.net/projects/texstudio/files/texstudio/TeXstudio%20SVN/ (warning: this is still experimental).
Since there is no feedback, I consider this fixed. Please report back if there are still issues with Qt5.