Sorry since I bring the table in AS another name you have to update all of the references to it like:
SELECT categories.catID, categories.catTITLE, link.galleryID, galleryRand.domainID, galleryRand.galleryIMG
FROM categories
INNER JOIN link ON categories.catID = link.catID
INNER JOIN (SELECT gallery.* FROM gallery ORDER BY Rand()) AS galleryRand ON link.galleryID = galleryRand.galleryID
WHERE galleryRand.domainID = 1 GROUP BY categories.catTITLE