Why does the plugin create more than 1 sitemap?
Image Sitemap Pro plugin creates separate sitemap files in two scenarios
Image Sitemap Pro contains 1,000 URL per sitemap.This is to ensure sitemap file size is less.Smaller file size helps Google bot read file faster. So if your website has more than 1,000 URL you will see more than 1 sitemap file.
Image Sitemap Pro creates separate sitemap files for eCommerce plugins and Gallery plugins. So if you are using these two together on your site then you will see more than one sitemap file.
Along with regular sitemap file it creates one index sitemap file.The index sitemap file should be added to Google.All regular sitemap files are included in the index sitemap file.
Which sitemap map should be added to Google?
You should always add the index sitemap file for example your-domain.com/sitemap-index-image.xml
Can I use your plugin with regular sitemap plugin?
Yes you can this plugin with regular sitemap plugin.Image sitemap pro deal with images only.So both sitemap files play well together. Here is link stating Google point of view on duplicate URL in sitemaps
File is not writable error
If you are getting file not writable error then make sure your web root is writable. You can create below blank files and make them writable (664 file permission)