WordPress has an in-built image gallery feature which does not yet seem to be common knowledge even though it’s incredibly simple to use.
So let’s say you are writing a post that you want to include an image gallery in.
1) Using the usual upload/insert image button, upload any images you want to include in the gallery BUT make sure to just click “Save All Changes” instead of “Insert Into Post.”
2) If you want your gallery to have captions, make sure to fill in the Caption field when uploading your images.
3) To display the images you just uploaded, simply use the shortcode: