you'd click on one of the "+"-Signs and select
If you're using classic (and you do, if you really have a wiki-page) you place the cursor where the image should be inserted, the go to ribbon -> insert -> picture
Click a picture
You can not "enlarge" a picture in SharePoint-Standard. However, since the picture resides on the SharePoint, you could add a hyperlink to the source like this:
- Upload the picture to SharePoint
- Edit the Wiki-Page
- Ribbon -> insert -> Link -> from SharePoint
- Chose the picture. Now the name of the picture is a hyperlink to the picture.
- place the cursor in the middle of the name
- Ribbon -> insert -> picture -> from SharePoint
- Chose the picture again.
- remove the name-characters that surround from the wiki-page.
Now you have a clickable picture.
If you do not have a place to store the pictures
- gear-symbol -> add an app
- "picture library"
- add images there...