Depends to some extent on your email application, some give you the option to download attachments with a single click, e.g. Mozilla Thunderbird lists attachments at the bottom left corner of the window, if you run the mouse over a file name it gets highlighted, left click and it downloads, other email apps may list attachments elsewhere on the window. In some cases you have to right click on the image or video and select either "Save as" or "Copy" from the pop up menu.
If the image or video is a link, open the link and highlight the URL on the address bar and copy it and paste it, if pasting it to a post here click on the icon of a globe with to links of a chain beneath it, if it is an image location click the icon to the right of the globe/link one, if a youtube video then the one next right to that.