Black or colored borders may be embedded in a thumbnail or added by the page displaying it. Knowing which case applies prevents unnecessary cropping and incorrect assumptions about the source.
Open the image by itself
Use the direct image link from ToolNivo and inspect the file outside the preview container. If the borders remain inside the image boundaries, they are part of that downloaded file. If they disappear, the surrounding layout created the extra space.
Decide what the border does
An embedded border may preserve a differently shaped source or form part of the design. Removing it changes the composition and may reduce the available dimensions. Check whether text or important details sit close to its edge.
Preserve the reference
Save the original before making any derivative crop. Record the changed dimensions when the result will be shared with another editor. A clean distinction between source and crop keeps later reviewers from believing the altered framing was the creator's published original.