The first thing you will need is a TIFF image of the artwork you would
like to use. The TIFF image should be a 24bit (true color) color TIFF
image.
Now you have one decision to make:
1) Will the texture completely cover a single face of the model
or
2) Will the texture be just a small area in a larger face.
In the case of the nail gun the texture is intended to fix exactly over
one face of the model. This simplifies things.
For case 1).
You need to create a texture in UG/Photo using the TIFF image.
When you edit the texture first select "attern".
The type of pattern will be "Wrapped Image".
Click the Filename button and select your TIFF image.
Now you need a texture space.
While you edit the texture select "Texture Space".
Set the type of texture space to Arbitrary Plane.
Now you need to know the size of the area for the image
measure the width of the area in UG and set that as the
scale value for the texture space.
Measure the height of the area for the texture. Set the
aspect ratio as " height / width ".
Set the origin of the texture space to be the upper left
corner of the area.
Set the up vector to be in the direction of the "height" of the
face
Set the normal vector to be normal to the face.
 ress OK to finish the creation of this texture.
Now that the texture has been defined as a wrapped image with an
appropriate texture space to scale an position it - we are ready to
assign it. Go to the Assign Materials/Textures in UG/Photo and
assign this texture to the face. Click on the little option button to
make sure you are picking faces. Press OK or APPLY.
(You probably also want to assign a different material to the face
as well. I tend to use some plastic material. The color of the
material
will be ignored in favor of the colors provide by the TIFF image in
the texture - but the reflectance will still come from the material.
By having a separate material for the logo you can better control
the brightness of the logo without changing the appearance of other
geometry )
Now you should have it - a texture applied to a face of the object
and the logo should be properly scaled for that area.
For case 2) One method to use is to create a rectangular surface
that is slightly offset from the face. If the face is curved then I
tend to project curves onto the face, move them off the face slightly
and create a free form surface with those curves.
Now that you have the offset face, you can add the image texture
in the same manner as case 1.
Other techniques include: splitting or trimming the original
face so that you have region for the logo.
John Baker mentioned using stencils. Stencils are mainly useful when
the logo is being applied to a glass bottle, or when the the logo spans
multiple faces which have different colors. Stencils almost always
require the creation of an offset surface.
:I 
attern".