FAQ:How can I create a thumbnail file?
Thumbnail files for assets, are PNG-files that are renamed from filename.png to filename.thumb. The default resolution is 128x128, though another resolution should work, too. Of course, the thumbnail file must be in the same folder as the asset and must be named equally to the asset (except the filename extension). The filenames are case sensitive (Filename.thumb and filename.thumb will refer to different files and e.g. Filename.thumb will not work with filename.mhclo). You can use the built-in renderer of MakeHuman to create a thumbnail of your asset, set the resolution to 128x128 and save the image as PNG. Than use your preferred file manager to rename and move your newly created thumbnail file.