[Turtle91]

Image order inside of the folder/zip doesn't matter as long as the filename is correctly referenced in the link. It only matters when you open it in an editor program. IIRC both Sigil and Calibre Editor order the images numerically/alphabetically (0-9A-Za-z).

01-img.jpg
02-img.jpg
Bravo.jpg
Delta.jpg
alpha.jpg
charlie.jpg

Roger mentioned you can bulk rename in Calibre, but you can also bulk rename using Sigil. Simply highlight all the images you want to change in the Images folder of the Book Browser window, then right click on one of them and select "Rename" - or type Ctrl-Alt-R - then Sigil will ask what you want to rename it to. It will automatically increment each of the files you have selected (if you have multiple selected, it wont increment if you only have one selected). eg. Img_ becomes Img_1, Img_2, Img_3 etc. If you wish to give it a specific starting number or format you can. eg. Img_010 becomes Img_010, Img_011, Img_012, etc.

I hope that helps!

EDIT: just realize they will be incremented in the order that they appear in the images folder - from top to bottom. You must individually rename them (1.jpg, 2.jpg, 3.jpg) so that they appear in the order you desire...then do a bulk rename and they will stay in that order.