Half-letter zine pages, imposed onto letter paper in saddle-stitch order. Fold, staple, done.
PNG, JPG, WebP or GIF. Single pages are portrait; spreads are one landscape image covering two facing pages (exact pixel sizes for your printer are under Make your images this size) — Stapler splits each spread across its two facing pages, even when they print on different sheets. Files are added in filename order — name them 01, 02, 03…. A spread placed first becomes a wraparound cover.
Before running the whole zine, print one sheet double-sided. If the back comes out upside down, flip the Rotate back sides switch and reprint. Printers disagree about which edge they flip on, and no setting is universal.
In the print dialog choose two-sided and flip on short edge. Set scaling to 100% or Actual size — "Fit to page" will shrink everything and throw the fold off centre.
Keep the sheets in the order they came out, stack them, and fold the whole stack at once along the centre. Folding sheets one at a time makes the spine creep and the edges stagger. Pages print right up to the fold so spreads meet across the gutter.
Two staples on the fold, roughly a quarter and three quarters of the way down. A long-reach stapler makes this easy; a regular one works if you open it flat and staple onto an eraser or a folded towel.
Everything happens in your browser — the images never leave this device. Page count is padded to a multiple of four because every folded sheet makes exactly four pages; that is a property of the paper, not a limitation of this tool.