When creating a post or a page, click on the ‘Add an Image’ icon, next to the ‘Upload/Insert’ text.
Select and upload your images all at one time. (To select multiple files on your computer, lasso them with your mouse or use shift-click to select a range of files.)
When all of your files have uploaded, click ‘Save All Changes’.
You should be taken to the Gallery tab, where you will have some further options to choose from. When you’re happy, click on ‘Insert Gallery’. The image gallery should be inserted into your post or page.