I've found what I was doing wrong.
"Hard" save my Base64 string (using ``) in datafield was my problem. On "on load" form Maestro had problem with template string and change it in same way. When I load file from server after form is loaded, problem disappears.
Thanks for advices :)
Check that your src attribute is formatted correctly with 'data:' etc. Refer to http://stackoverflow.com/questions/1207190/embedding-base64-images
Can you provide link to form or some example code of what you tried? That approach should work.