First I think you have to associate the TakePicture with the Account entity and you can check the system owner systemMember to save the user who created the object
for ClaimSubmit you have to create an association between the two entities instead of the image attribute making the association based on the relation that you need to achieve the business case that you have.
Hope this answers your question if yes please recommend this as an accepted answer
Should it be possible to have multiple pictures related to the same Claim?
Why don't you use an association between the two entities in stead of copying the fileid to the ClaimSubmit?
Information about setting up a REST service including image files can be found here.