Seems to have some problems when batch adding more than say 15 pictures. The first 10 Ok then the rest will be a red cross, and i have to refresh page to continue. Seems to be some time out but where
hard to say, maybe your images are too big, your server time-out to low.
Are you self-hosted or web-hosted. Give some more info on your server setup (OS, php version, image lib used etc) and on the size of images you're trying to upload.
As the batch-add feature greatly depends on server config there's no recommendation what number of files and what file-sizes should be batch-added in one go, but it shouldn't matter, as you already found out how to fix this: by hitting "refresh" in your browser.
The place to look for time-outs are: webserver config file (apache conf) and php.ini.
GauGau