File-Export is what I use when saving images as a flattened picture. I don't actually know if there's a difference between exporting and 'save as' but I opt to use export anyway haha. Although I do make it a point never to resize using CSP because it just makes my drawings look weird.
The way I put pages together isn't probably the most efficient way, but I'm really particular about spacing in between my panels. So what I do is:
- Open a new canvas file. In my case, I use 20000 (Length) x 24000 (Width)
- Paste all of my panels on the canvas
Through CSP, I honestly don't think there's a shortcut to merging all files into one vertical format. Maybe photoshop has something like that. But CSP, afaik, isn't optimized for making a vertical webtoon. :(((