Can it be that thumbnails are not computed for images larger than 500 KB?
Every file that is smaller than 500 KB gets a thumbnail, the others show the default icon. I tested with PNGs and JPGs so far.
Yes, that's true - system does not generate thumbnails for images larger than 500kb. You can read why activeCollab behaves this way and what you can do to change that behavior here: Problems with memory and thumbnail creation.
Every file that is smaller than 500 KB gets a thumbnail, the others show the default icon. I tested with PNGs and JPGs so far.