I have not used the feature yet, but their is a discussion of this component here Signature not displaying in Image component there was a bug at the time which may be fixed by now about displaying that in Adalo. The text string you see is base 64 encoding of the image data. You will need to decode that into an image to have a jpg. https://codebeautify.org/base64-to-image-converter is one place I found with Google that does that there appear to loads of options.
This is not a total answer to your question but some info to shed a bit of light