There are like 100 online manga viewer sites, of which OneManga is undoubtedly the very worst. Shit quality, shit watermark (they removed it?), shit advertisements, shit policies...
I'd rather use some of the others, which offer the very original images, preload all of them, allow navigation without reloading the page, have no ads (for those who still haven't blocked them), and have a fucking download link right there.
By the way, your code sucks. I know because it's more than 10 lines long, and the fucking filenames for the images are sequential. They can be downloaded with a single line of bash script. I'll give you 10 lines because of non-bruteforce double page handling, which is for pussies.