A representation of a coin or other money on sign work is usually not a problem. The standard for
trademark infringement is the "likelihood of confusion" on the part of the viewer. No viewer will likely mistake a picture on a sign for legal tender.
A point to consider, though, is regarding
copyright infringement.
Most stock image-licensing services pointedly forbid the use of their images as logos or as part of a logo or branding scheme. The reason is simple: the images are protected by copyright. Copyrighted images owned by someone else may not be used in logo designs for your client—this would be copyright infringement.
Further, the US Patent & Trademark Office would never knowingly grant a trademark registration for a logo that contains clipart. Unless, of course, you create the artwork yourself, or, if you purchase an exclusive license for a piece of artwork, which is sometimes available from the original artist.
This is from the
Shutterstock license agreement on prohibited uses of their clipart [see sec. 1.2, par. h, under PART 1 VISUAL CONTENT LICENSES]:
"Use any Visual Content (in whole or in part) as a trademark, service mark, logo, or other indication of origin, or as part thereof."
Prohibited uses for Shutterstock images
Brad in Kansas City