Welcome to Android Central! Which phone do you have? Most devices have a clipboard that only remembers the last thing you copied to it. So if you copied a number of items, only the last one is remembered. To paste whatever is in the clipboard, go to any empty text field, long-press and then Paste.
The Gallery will only display image files that have been saved to your phone's storage. How did you copy an image to the clipboard? Usually, if you long-press an image on the web, you'll get options to Download or Save, not Copy to the clipboard.