I don't think this can be done because FileMaker Go. can only import from a fmp12 file. You could move the photos and file to a computer and then import all of the photos from a folder. You could use a looping script with an insert but the user would still need to pick every photo individually.
You can not import a group of pictures. You can insert one photo at a time using insert from device : type photo library. You could create a loop script with a dialog box asking the user if they would like to import another photo. The user has to select the photo from the photo library.
Will your solution be hosted on Filemaker Server, or just used locally on the devices?
The nice thing about dropzone.js was that as well as enabling drag & drop of multiple photos from Finder on Mac or Windows Explorer, it also allowed multiple photo selection from iOS photos!
The 'dropzone' itself is loaded into a Web Viewer within the FileMaker solution.... In FileMaker Go, the user just taps the 'dropzone' and the iOS photos selector pops up... Works great, but there was definitely a bit of trial and error to get everything working nicely. Another bonus was that it also works in WebDirect :-)