You can easily encode a file into Base64 text with Window's built-in (post-XP) certutil.exe utility. Copy/paste the following code into an empty .txt text file, rename it with a .bat extension like "Base64encode.bat", then put the bat file into your Windows "SendTo" folder. The latter is normally located at ..\Users\(username)\Appdata\Roaming\Microsoft\Windows\SendTo.
echo The file will be encoded to Base64 txt
certutil -encode %1 %1.txt
Then, to encode a file into its Base64 text string, right-click the file and select the bat you made from the SendTo context menu. A file will be created in the same folder as the original named with the original filename and a .txt extension, like "artwork.jpg.txt." The original file will be kept. Opening the .txt file and copying everything BETWEEN the "---BEGIN CERTIFICATE---" and "---END CERTIFICATE---" lines will allow you to paste the original file's Base64 equivalent anywhere you want. I use this method now when adding custom images to MusicBee skins' "NoArtwork" display element (https://getmusicbee.com/forum/index.php?topic=22889.0