After you've done file-> export -> image,while waiting you are not selecting a new window in the GUI or moving on to other window are you ???
Because,if you move on to a window/change window while the export is taking place your image will contain information about the newly selected window.This is the only reason i can think as of now.
Until this problem is resolved,if you just want to see waveforms in images,why don't you snip them using Snipping Tool in Windows.