A browser's Save page command can store HTML rather than the image you wanted. The result may look like a download success while leaving you without a usable thumbnail file.
Open the image directly
Use ToolNivo's Open image in a new tab link for the selected resolution. Confirm that the tab contains the image without the tool's form and navigation. Then use the image-specific save action offered by your browser.
Inspect the file type
Open the saved item in an image viewer. A webpage file may have an HTML extension or open a collection of page assets. Do not fix this by simply renaming it to JPG; changing a filename does not convert its contents.
Repeat from the correct surface
Return to the image tab and save again if necessary. Keep one verified copy and remove accidental duplicate webpage downloads when they are no longer needed. A quick file check now avoids confusion when an editor later refuses to import what appeared to be an image.