I recently ran in to an issue using the file manager in Nemo. None of my images where showing any previews / thumbnails. If this happens to you, do the following.
Open the folder containing your images
In the top menu goto Edit >> Preferences
Click on the Preview Tab
Under the section headed Previewable Files, select the maximum file size for your images to be shown. In my preferences thumbnails are displayed for images 10MB or less.
I hope this resolves your issue. This is especially helpful for me as I take a lot of photos on my mobile phone, and wasn't able to view any previews when my phone was connected.