"Fill paper" = no white borders, edges may be cropped (recommended for photos).
"Fit whole image" = whole image visible, white borders if needed.
⚠ For borderless 10×15 prints
In the print dialog: set Margins → None, Scale → 100%, and uncheck "Headers and footers". In your printer driver, choose the borderless paper variant (e.g. "10×15 cm Borderless" on Canon Selphy). Otherwise the printer adds margins of its own.
How printing works
All images stitch into one print job — your browser shows one dialog, click Print, all pages send. Brightness boost compensates for dark photo prints (try +15 to +25 for Canon Selphy / dye-sub printers) and only affects what's printed, not saved files.