To “download” an image that you can not download because the website prevents it, you can use OCRExtractRelative. Make the pink box around the captcha and the green box anything nearby as anchor => Now the _lastscreenshot image in the Screenshots tab has the image you need. Use hard-drive mode, then this image is directly on the harddrive. Now you can grab it from there and upload to any service or website.
See here: How to Scrape Pop-Up - #9 by ulrich
Once you have the image, you can use e. g. XRunAndWait to run e. g. a Python script that does the base64 conversion (Similar to here: Allow user to select file name (Open file name picker dialog) - just instead of opening the file picker, you would do the base64 conversion and captcha breaking)