Miraheze provides imports for all wikis. To request an import please go to Special:RequestImportDump and fill in the form. You can read Moving a wiki to Miraheze if you need help creating a dump file.
If you wish to also import images, please create a Phabricator task and use a cloud provider (like e.g. Google Drive or Dropbox) to link us to your images. If it is a private wiki you can send this by email to sremiraheze.org.
Imports are done by the System administrators. They are done using the importDump.php and importImages.php maintenance scripts. The duration of the import depends on the size of the wiki.