A downloaded thumbnail is normally a flattened image. It does not contain the original layers, text objects, masks, or source photographs that made the artwork editable.
Understand the limitation
Changing a word in a flattened file may require rebuilding the background beneath it. Moving a person can expose missing pixels. These tasks are different from editing the original layered project.
Organize two kinds of source
Store the original design project, fonts and permitted assets in a master folder. Keep downloaded published images in a reference or export folder. Use clear names so an editor does not mistake a final JPEG for the editable master.
Rebuild carefully when needed
If your own master is lost, use available originals to reconstruct the design rather than repeatedly patching the compressed export. Keep the recovered version labelled separately. For someone else's artwork, downloading the image does not provide its working files or permission to recreate and reuse the design.