Welcome,
what other URL you expect it to have? 'localhost' is the default Domainname which always resolves to the own machine. No one else on your network can use this to get to your wiki. Therefore you need a name which is known to all computers on thr network and always resolves to your machine. If you are running a private network with a DSL router, this MAY be done by your router if you use your machines hostname. So give it a try and use
http://gandg-pc/dokuwiki and replace 'gandg-pc' with your real hostname, though.
Uploading image size is controlled by a PHP setting also. See here for details
https://www.dokuwiki.org/faq:uploadsize. Try to locate the php.ini file inside XAMPP directory and edit or add this settings
upload_max_filesize = 15M
post_max_size = 15M
There may be another limit inside Apaches request body in php.conf inside apache directory. Look for 'LimitRequestBody' and change it to eg. '0' tomhave no limit.
A third location may be a .htaccess file inside dokuwikis root dir. This can override limits set in global apache oder apache-php config files.