Some groups aren't very particular about numbering, but no matter how you number the pages, I really recommend keeping it consistent. Also, most groups will synch their numbering with the actual numbers on the manga pages, which cuts back on confusion. A common format is:
You can bump covers to the front by putting two dashes after the volume number: Gin_no_Yuusha_v01__cover.jpg
And credit pages usually are page 000: Gin_no_Yuusha_v01_c01_000.png
If you use that system consistently, the reader can extract all chapters of all the volumes to one folder, and they would read in the correct order. Zip files would also be named similarly: Gin_no_Yuusha_v01_c01[groupname].zip.
Saving to PNG ->