Byte arrays and Base64 are not the same thing: Base 64 is a "stringified" representation of binary data, which uses a limited set of 64 printable characters (hence the name) to represent the same data in a way that can easily be transferred over text-based links such as HTML.
Byte arrays contain the binary data directly, and that means full eight bit data in an array.
If your method expects Base64 data, you cannot hand it binary image data and expect it to work, any more than I can hand you a copy "War and Peace" in the original Russian and expect you to read it!
If may be that you can use Convert.ToBase64String Method (System) | Microsoft Docs
] to convert it, but since I have no access to your data, I can't tell from here if that will work - it will depend on the content of the byte array.