I’m just trying to understand the issue, one way or the other and I apologize that I am still a little unclear. Please be patient while we troubleshoot the issue.
I couldn’t find much info on “Error: -300, Message: IO error“, certainly nothing specifically related to WordPress, themes or plugins (http://support.dev7studios.com/kb/nivo-slider-wordpress-plugin/using-custom-themes is too broad to be of any help here – please note that the theme does support featured images anyways and seems completely unrelated to issues of uploading images). I also do not experience this issue myself in any browser nor has there been any other report of this issue yet (so this is likely an isolated issue).
My best guess is that it’s a PHP upload limitation (especially if you’re trying to upload many images at once), in which case you’ll need to contact your host support.
– What hosting/kind of hosting do you use?
– Is everything up-to-date? (check under Dashboard > Updates)
– Have you tried deactivating all other plugins except for Nivo Slider? (to see if one of the other plugins is causing the conflict)