I realized that if I uploaded larger ones they were viewable by me (and admins, I presume), but I never noticed being able to view any other photos full size. What exactly causes the "larger image" link to appear on the node/view page?

you have to put in yourself. After you post your image, click it to get to larger size, then copy the url

return to the main image page, click edit, and in remarks type

(here I have added asterisks to prevent the code from executing):

[url*=paste in the url you just copied]larger image[*/url]

at least that's the way I do it

So you're just adding it manually, got it. Since I eventually get around to uploading my pictures to Wikimedia Commons after they're ID'd, I suppose I could just as easily add a link to there... thanks for the idea!

somebody else on BugGuide told me about it