Two windows in the Live Page Builder protect your work and your pictures. Restore unsaved changes? brings back work you did not save, and the Media Library window lets you pick a picture you already have, or upload a new one.
Read it when the builder asks you about unsaved work, or when you want to reuse a photo.
Step 1. When the builder asks to restore
If you closed the tab or lost the connection before saving, the next time you open the builder on the same device you see Restore unsaved changes?. It says when the work was done and how many sections it holds.
Step 2. Restore or discard
Press Restore to get that work back. You see Unsaved changes restored. Press Discard to drop it and start from your saved draft.
Step 3. Save the restored work
Restored work is not saved yet, so the save line says Unsaved changes. Press Save draft at the top, and wait for Saved just now.
Step 4. Open the picture choices
Click a picture on the page, or an empty spot that says Drag a photo here, or click to choose. Choose Use existing from Media Library. To take a new photo from your computer straight away, choose Upload new instead.
Step 5. Browse your library
The Media Library window opens on Browse. Search by file name, and click the picture you want so it is selected.
Step 6. Or upload in the window
Open the Upload tab and drop your file, or click to choose it. The line says JPG, PNG or WebP up to 10 MB. The picture you upload is selected for you. If you see Already in your library; the existing file was used., the same picture was already there.
Step 7. Use it
Press Use selected. The picture takes its place on the page. Press Cancel to close the window without a change.
Result
No work is lost when a tab closes, and every picture you use comes from your Media Library, where it stays for next time.