The uploader retries on its own
Photos go in small batches. The uploader sets aside batches that didn't make it, and after the first pass it comes back to them and retries up to six times, waiting longer each time. On a shaky connection you'll see the bar pause near the end with "Retrying upload of ... photos", and then finish.
What an upload won't survive
- closing the tab or going to another page,
- your computer going to sleep,
- signing out of the panel.
Your browser runs the upload, so when the page goes away, whatever hasn't been sent yet goes with it. Photos sent earlier stay in the gallery.
After an interruption, upload the same folder
Click Add photos and select the whole set again. The upload starts by itself. Files that are already there get recognized and skipped, and only the rest is added. You don't have to guess which photo it stopped on.
When the same error keeps coming back
If the upload fails at the same spot every time, it's usually one specific file, not your connection. Try uploading that file on its own: you'll see the exact message, for example that the file is too large or the type isn't supported.