Use this widget to display base64 image-https://marketplace.mendix.com/link/component/112248
If you want to store the image in a string using base64encoded, you should change the Issue_Signature Entity (and remove the underscore in Entities please, they might be confused for an association) by removing the generalization to System.Image. and add a String Attribute like ImageString.
I hope you chose for base64encode because you can’t handle the System.Image binary. This could be the case if you expose that data using an REST API. Otherwise, you can take the base64encode and -decode out and just use the default binaries. If you do it for security, you should use encryption and decryption. That can be used for the binary as well.
Sticking to base64encode:
If you need metadata that you now get from the generalization, you should add them. (name of the person of the signature etcetera).
Create persistent Entity SignatureImage with generalization with SystemImage as a placeholder for the uploaded image (no associations). This is the source for the MF that will base64encode the Image to ImageString. Don’t forget to clean up the SignatureImage Object after the conversion to string, or do not commit it at all. Otherwise, you could end up with stored Images that you may not want.
When you want to sign, do the opposite. Retrieve the SignatureString; create an object of SignatureImage that you populate using base64decode Action.
I could help with a littje app doing exactly that if the information is not sufficient.
Go Make It