My previous answer below is incorrect.
Assuming the work is made under the laws of the USA:
As Horatio points out in the comments, a work made for hire is not considered to be a work of visual art:
A work of visual art does not include—
(A)(i) any poster, map, globe, chart, technical drawing, diagram,
model, applied art, motion picture or other audiovisual work, book,
magazine, newspaper, periodical, data base, electronic information
service, electronic publication, or similar publication;
(ii) any merchandising item or advertising, promotional, descriptive,
covering, or packaging material or container;
(iii) any portion or part of any item described in clause (i) or (ii);
(B) any work made for hire; or
BUT according to Circular 9: Works Made for Hire, a work that you commissioned from an independant contractor is a work for hire only if it meets one of the 9 criteria listed in that circular and if you have a written agreement that it is a work made for hire. In your case, if you are just reselling a contractor's work as is, it is not considered work made for hire and you do not own the copyright by default.
The author of the work - your contractor - retains the copyrights for the work unless he transfers them to you, so you must have a separate clause in your contract/purchase agreement requiring the contractor to explicitely transfer the copyrights to you.
However, as you might have noticed above, drawings and graphics of commercial nature are not considered "works of visual art", but instead fall under the category of "Pictorial, graphic, and sculptural works", which are also protected by copyright but in the US do not provide additionnal moral rights such as the right to claim authorship.
If my understanding of this is correct this time around, US copyright law does not provide moral rights to the author of a work of pictorial, graphic and sculptural art, which means no one gets the right to attribution and you do not have to formally acknowledge the author.
If you are outside of the USA, things are different when it comes to moral rights so you have to check what your country's copyright. For instance, in France, the author retains moral rights (attribution, etc) forever and can not transfer these to you.
[My previous answer]
No, you can't:
According to article § 106A . Rights of certain authors to attribution and integrity,
(a)...the author of a work of visual art shall have the right to claim authorship of that work, and...
(b) Scope and Exercise of Rights.—Only the author of a work of visual
art has the rights conferred by subsection (a) in that work, whether
or not the author is the copyright owner. The authors of a joint work
of visual art are coowners of the rights conferred by subsection (a)
in that work. ...
She has the right to claim authorship for her designs, unless she explicitly waives this right for each design in writing.
You do not however have to provide a link to her website or even mention her company name. These authorship rights belong to the actual author of the work, not the company for which the work was done.