Most times you set the number of rows and columns, and the app autofits the images accordingly and outputs the result as a new file...
Go to the top of the class, that man!
Look for a small DC credit... You're right, it works in IrfanView. For some reason, if there's more than one page, IV won't output PDFs as one file, it prints each one separately, but for my current purposes that's OK. IV can
save multiple pages as JPGs without user intervention, though.
For anyone wanting to know how: IV, press "T" for thumbnails. Mark the images you want. Alt-F, go down to "Create contact sheet from selected files." Look for "Image parameters." Pick the number of rows and columns etc. you want, press "Create," up comes the Print Preview, and off you go.