disneymoviesandfacts:

Every time Pleakley looks at or attempts to show someone information about Earth, the device used is a “Viewmaster”. Viewmasters create the illusion that the viewer is looking at a three-dimensional image that’s actually created by each eye looking at a slightly different pictures. Its basic premise is that the person looking in it has two eyes to create the effect, but Pleakley is a one-eyed creature and therefore would not see the 3-D effects of the Viewmaster. Adding to the gag is when he hands the ViewMaster to Dr. Jumba - a one-eyed alien handing an four-eyed alien a viewing device intended for two-eyed humans.

So a bunch of things. One: anyone wonder how odd it is that he would have an object from earth, let alone one for two eyed beings?
Well, we all know his fascination with mosquitoes.
Now here’s hte thing, at one point, mr. bubbles says he saved the earth once by convincing an alien race that mosquitoes were endangered and needed to be protected. pleakley seems to have been familiar with earth even before they arrived. My theory is that the viewfinder was given to him by Mr. Bubbles and that’s why the only slide is about mosquitoes.