i hope i am understanding your objectives correctly, (it's late in the UK)
assuming you are in the 'protected page', you know the userid of the person logged in
on my voting form, i would have a recordset showing the images from tblimages, it doesn't need to be joined to anything, just show all of the pictures.
by selecting a picture, you would then know it's imageid.
posting the form would insert a row into tblvote adding userid and imageid, so you know who has voted and which image they have voted for