The way I see it, this basically boils down to a question of what is a layout and what is an actual piece of intellectual property. First off, we don't even know what the layout of this sign even encompasses, because the entire image was not shown, only the relevant portion with a photo of a face and an added effect, plus a portion of what appears to be a letter Z outlined in purple. I highly doubt there's any real intellectual property rights being infringed upon here. The OP even states this in his original post, "I am having to copy a political sign LAYOUT originally done by another shop". I'm sorry, but to those who think that laying out a purple background, a head shot with a Photoshop effect and some white text with a purple outline added for contrast is copyright infringement, I disagree.
From LegalZoom...
"A layout design is not copyrightable, but a work encompassing layout design, such as a catalog, may sometimes be copyrightable as a compilation of photographs, artwork and text."
So unless someone can clearly establish what element here is cause for legal concern, this appears to be just a layout, a simple arrangement consisting of a photo and some text. Typesetting and arranging photos doesn't appear to constitute any level of infringement from what I can tell. In my opinion, unless a sign design contains original creative elements, it's fair game to reproduce. If the photo had been significantly altered from the original photo by the previous sign shop, then sure. But adding a fade and an outer glow in Photoshop? C'mon now.
Again, from LegalZoom...
"Logos can be copyrighted if they contain the required amount of creative expression. For example, a logo consisting of an original illustration of a fish would be copyrightable; a logo that consisted of a red letter A in a particular typeface would probably not be copyrightable."
This further reinforces the notion that unless a work contains a significant amount of 'creative expression', as they term it, it can't be, or at least 'probably' can't be, considered a copyrighted work.