|flash-drives/pen-drives are the wrong media type for your requirements as they are most unsuitable for precious photo images for your clients owing to their poor reliability - you will get a barrage of complaints from your customers.|
The medium to use is high quality CD-R or DVD-R. These are much more reliable and the files you put on them are inherently read-only without you having to do anything.
You can also write a meaningful title on the disc (using only a cd-marker pen), something you can't do with a pen-drive.
Also with a CD, FAT32 or NTFS is irrelevant. All you need to decide on therefore is the image format to use for writing to CD. For cross-platform between Windows & Mac, the best option is JPG.