Leo,
The preview image is in the resource itself.
These resources (A:M files) are text files. For instance, a material file (.mat) has a section/container in the file where the image data is stored.
As such the preview images simply do not exist in the ones that are showing the default material preview image. They'll need to be manually added.
The steps to place image data into an A:M File currently varies depending on which resource you are using.
Project files and model files you should be able to Right Click and [Create Icon] or [Create Preview Image]* and the preview image will be added to the file.
With materials this doesn't seem to be the case at this time.
- Go to File Info in the properties menu and click on the Preview image.
- Select the image you want to assign to the file
I've found that using a program like
Irfanview can crop and save images very quickly for this purpose but any graphics editor will do.
Having said all this, if the files in question can just be replaced with more current versions that already have the preview image in the file... even better.
I suspect most of the materials with preview images you are looking for can be found here:
Extra CD Volume 1 MaterialsNote: I have noticed that on occasion saving a material will generate the preview image.
It may be that these materials files already have the preview image but not in proper format.
Perhaps when saved the material file is updated and the preview image is fixed?
You can usually tell whether a material has a preview image inside by looking at its size.
Material files with images are much larger than those without (but all are quite small).