Are the files correctly displayed in Lister?"Get from Irfan-/Xnview for:" - work only for *.bmp files. (Xnview is installed)
If yes, they should work as thumbnails, too.
If they are shown in Lister but not in Thumbnails, try disabling the thumbnail database (by clearing the path to it).
If they are not shown in Lister try adding IviewAdditionalTypes=*.jpg to wincmd.ini under [Lister] (see also Help, section 4b.)
HTH
Roman