Irfanview is a free imaging program I use.
There is something about your thumbnails that SimpleViewer does not seem to like.
When using one of your main images as both a thumbnail and main image, both are displayed by SimpleViewer.
When using one of your thumbnails as both a thumbnail and main image, neither are displayed by SimpleViewer.
However, if I open one of your thumbnails in my imaging program and simply resave it (keeping it as a png file), then it is displayed OK.
What program did you use to create your thumbnails? Are there any settings you could change whilst saving the files?
I realise that my suggestion (to resave the images in a different imaging program or convert them to jpgs, perhaps as a batch process to save time) is a workaround rather than a solution but I do not know why your thumbnails are not being displayed.
I have tried creating png files with different encoding (ASCII vs binary), different compression (with and without PNGOUT plugin) and with and without transparency and I cannot make a png file that SimpleViewer fails to display.
Perhaps someone else can shed some light on this.
SimpleViewer Support Team.