If I copy an image (eg. in Okular, open a PDF, right-click drag, and select to copy as an image to clipboard) that is much taller than it is wide, the preview shown in the popup (the one that appears when you left-click the clipboard system tray item) is scaled to fit width. This means I have to scroll very far to reach the next item. Perhaps you could consider making sure this image preview is scaled in such a way that it does not go out of the popup, and thus one does not need to scroll too far to reach the next item.
Fixed with https://commits.kde.org/plasma-workspace/7c644c4499c89d026c12a404a758f37f50303948 The preview images in the Clipboard popup are now always square. See also https://phabricator.kde.org/D10557