Currently that was not possible to convert to use dash because all the existing image file name already convert it to use underscore.

May i know did it show up any warning or error if the page contain those underscore link in SEO grading tools?

Based on what i checked this thread from Google https://support.google.com/webmasters/answer/76329?hl=en , they only mentioned recommend to use dash for the URL.

Yes, if google recommends it, that is what we need to do. I would delete and reload each of the galleries if there was a way for it to use the dash instead of underscore.

Hello Tony,

It's not entirely possible to change this behavior because file names are automatically prefixed with underscores. Anyway the structure that was generated, makes sense as it is a distinct separator between underscore and words.

For instance, take the following file name,

b2ap3_large_Eric-Church-Breaks-Record-2.jpg

The underscore portions are used internally by the system and ideally it shouldn't be grouped up as a "word" in SEO. Therefore if Google ignores the "b2ap3_large_" portion because it uses underscore, it's perfectly fine as you want them to index "Eric Church Breaks Record 2"