Catherine & Eleanor's perusal of Mrs. Tilney's poitrait wasn't interupted at all, it wasn't until afterwards when they were going to Mrs. Tilney's rooms that they were interupted by the General who wanted Eleanor to answer a note.
Catherine's reaction to the picture is in Chapter 24 of NA: "It represented a very lovely woman, with a mild and pensive countenance, justifying, so far, the expectations of its new observer; but they were not in every respect answered, for Catherine had depended upon meeting with features, hair, complexion, that should be the very counterpart, the very image, if not of Henry’s, of Eleanor’s — the only portraits of which she had been in the habit of thinking, bearing always an equal resemblance of mother and child. A face once taken was taken for generations. But here she was obliged to look and consider and study for a likeness. She contemplated it, however, in spite of this drawback, with much emotion, and, but for a yet stronger interest, would have left it unwillingly."