You should be able to get this working with a lookup field to that table of images. This would be a formula text field with html enabled.
"<img src=" & URLRoot() & "up/" & [_DBID_Table where the images are stored] & "/a/r" & [Related Image] & "/e22/v0" & " height=\"100\">"
Replace the 22 with the fid of the file attachment field where the signature image is stored. I have not tested that formula for syntax, so ket me know of there is a problem.
As for security, lets get the image working and then we will figure out security access.