Very often I'll be asked to do a henna design as a way to "test drive" an ink tattoo design ... before all the business with needles and peeling skin.
Does it work?
The Ins and Outs of Using Henna as a "Test Tattoo"
As I ranted here, henna and ink tattooing are distinct and different art forms, even though both leave you with a design on your skin. Besides the fact that henna is temporary and ink tattooing is (practically) permanent, certain designs can work well with both henna and ink but others do not.
Most henna designs are essentially line art: You have the "positive" space where the color is and the "negative" space where the blank skin shows. You can do shading with henna using a couple of techniques but they are somewhat limited.
Also, as I talked about in this post, henna comes in one color only: red/brown. The myriad colors of ink tattoos don't exist in the henna medium.
So, with all that in mind, a tattoo design like the skull one would also work very well as a henna design. The black would, of course, be reddish brown, but otherwise the design would be identical.
The snake design, however, is a very different story. The fine gradients of color in the design are impossible to replicate with henna. The best you could do would be to duplicate the basic design with limited shading ... not exactly what I'd call a "test drive".
Henna has its own motifs, traditions and history. Do an Internet search on henna images ... or better yet, check out my own gallery :) ... and you'll see that henna really is different, in a class by itself.
And then perhaps you'll want to try out henna for its own sake. :)