No sorry I cannot help.
(It might be possible for you to split it up, but I am not good with the base64 datatype, and the risk that it will fail is pretty big.. First you should try with a really small picture, see if it works. Then try a bigger one.)
Basically Photon do not really recommend large messages to be sent.
doc.photonengine.com/en-us/realtime/current/troubleshooting/faq
There is a setting in the photon server 'MaxMessageSize'. So if they do not allow the size you want in the Photon Cloud you might need to set up your own Photon server.
There is a specific Photon forum and a Photon discord you can join and ask how this can be achieved. This issue is not related to Construct, and you should ask Photon directly.