Multiple Upload lets you upload up to 10 images at the same time. It is provided by the MultipleUpload extension, written by Travis Derouin. Anyone who can normally upload a single file, can use Multiple Upload.
Go to Special:MultipleUpload on your wiki. Click to find the files on your computer and then fill in the summary and licensing information.
By default the files will upload to the same filename as they have on your computer, but you can change this manually if you wish.
When using Multiple Upload, the file upload size limit limits the total size of all the files combined rather than the size per individual file. The current limit is 10 MB for all of the files uploaded together. Thus, Multiple Upload is great for quickly uploading many small files, but not many large ones.
Admins can change the text at the top of the Multiple Upload special page using the system message MediaWiki:Uploadtext.