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Can you give us some more details please. What make and version Oscommerce module is used. What Xoops version is it installed on. Did you enable debugging and see relevant alerts?
For as far as I know it is along time ago someone ported Oscommerce to xoops, so incompatibility problems with newer xoops could occur. First I would check are the rights of the directory the images are stored/ rights on the images itself.