You've probably already looked at some of the galleries on the site, and now you'd like to upload your own photos. So to start, click on the Gallery link on the Add content page.
On this page you give your gallery a title, and you can also add a description so people can tell what it's about. Then click Save at the bottom.
You now have a row of tabs at the top, and you need to click on Upload New Items. On this page you can upload several files at once, so you can do all the files together if you want (you can come back and upload more later, so it doesn't matter if you don't include them all to start with). You can click Add files to use a file browser popup, or you can drag files from another window. When you've selected your files, click Start upload.
When the upload is complete (all the files have a green tick), click on Manage Items. Here you can organise the photos, in particular you can give each one a short title (to be more meaningful than "PIC0265.JPG"), rotate any that are the wrong orientation, and select which one is the gallery cover (the photo that appears on the page that lists all the galleries on the site). You can also delete using the boxes on the left, if maybe you've uploaded any by mistake.
Click Save when you're done here. If you'd like to re-arrange the order of the photos, click on the Sort Items tab. You can simply drag each photo up and down the page using the small cross icon on the left. As with the other tabs, click Save when you've finished.
That's it - you've published your photos in a gallery! Click the View tab to see your album as everyone else will see it (only you can edit it). It will also appear in the full list of galleries viewable by everyone.
You can use the tabs to go back to any of the previous steps any time to add or delete photos, change titles, or re-arrange. The Edit tab lets you change the gallery's title and description.