In Photos on iOS 10.3.3, when I select one picture and open the share sheet, there’s an option to “Add to iCloud Drive”. But when I select multiple pictures, the share sheet doesn’t offer that option. Adding multiple pictures to iCloud Drive one by one gets rather tedious. Is there some way I can add multiple pictures at once? Perhaps by using another app?
At present there is no easy way to do this, namely because Apple sees iCloud Photo Library and, depending on what you're trying to achieve, iCloud Photo Sharing as its iCloud solution regarding photos.
However, it is highly likely that with iOS 11 and the change to the new Files app, that you'll be able to do what you want, as Files will work more like traditional (if that's the right word) cloud storage. More details will be revealed at Apple press event on 12 September 2017, so you haven't got long to wait.
There is a bit of a messy workaround if you can't wait for iOS 11 or if your device isn't going to be compatible with iOS 11. This involves installing Apple's Workflow app and using it to Zip (i.e. 'make archive') of multiple photos and then move the zipped file into iCloud Drive.