If you don’t have an image gallery yet, you can create yours following instructions on Managing content until item 1 of description. When creating the folder, choose “Image gallery” in “Folder type”.
To find your image gallery, first find your user menu on top bar:
In user menu, click on Control Panel:
Then, click on “Manage content”:
Description
Find your gallery on the list and click to view it.
You will see your image gallery. Click on “Upload files” button
Select the pictures you want to upload and click on “Upload” button. If you want to upload more than 3 pictures at once, click on “More files”button and more fields will appear.