Directly save uploaded screenshots
I don't see a point of only uploading screenshots, or if there is one, having the option to only click "Upload and save" would be nice, as it has a much lower chance of accidentally not saving uploads.
Having the "Save" button available when adding source strings to it (after uploading) just baits the user into uploading the same picture over the current one, which deletes all the added connections.
If added source strings don't need to be saved, screenshots don't either. The proposed solution always works, both for adding screenshots right away, or adding source strings while doing so.
Edit: A bulk uploader would also be nice. First added all to upload without saving, and the second time around I kept saving every new picture over the one already saved. I think this depends on where the dialog is opened from.
Edit2: Will do a mockup later. I think just one list of images, with the added screenshots underneath, and a button to upload a new screenshot suffices.