The "art card" is simply the "Jonny Quest" logo that was used during commercial breaks in the stories when originally broadcast on ABC in 1964-65.

There were at least two distinct audio tracks for the art card. The first was placed between the main titles (opening credits) and the commercial for that week's sponsor. Its audio track featured Mike Road saying, "'Jonny Quest,' brought to you by ...," which introduced a 10-second spot about the sponsor. This version was repeated during the commercial break between Act 3 and the end titles.
Art Card Audio with General Mills sponsor message (12 seconds, 550K WAV file)
Art Card Audio without sponsor message (5 seconds, 136K MP3)

The art card also ran in the commercial breaks between Acts 1 and 2. Here, a short musical sting was heard instead of a voiceover. (5 seconds, MP3, 130K)

The art card was used in syndication in the 1970s. It was also included occasionally on some of the shows that ran on the Turner cable stations TNT, TBS and Cartoon Network in the 1990s, but only after the main titles. The voiceover was sometimes trimmed to remove "the brought to you by ..." phrase. None of the art card placements were included in the digital masters made by Turner in the mid-1990s and then shown on cable starting in 2000. The digital masters were also used (apparently) for the Jonny Quest DVD set released by Warner Home Video in 2004.

As members of the ClassicJQ.com forum pointed out, the art card bumper gives the title for the series. Without it, the viewer has to know the title in advance or guess it from the credit given Jonny's character during the main titles.