There is no doubt whoever obtained these photos have them uploaded on the Internet with malicious and deliberate intent. (Edison Chen)

Is whoever a plural pornoun?
Whoever is singular, and to be grammatially correct, the sentence should read has. And it should read 'has uploaded them'.

How about the acceptability of the following sentence?

Whoever want to, try to come up with one.
Students: We have free audio pronunciation exercises.
TeoWhoever want to, try to come up with one.
No. Can't do it. Use wants.
The questionof 'whoever' being sing. or plural depends on its use in the sentence, and if it is qualified by a noun in apposition:
Always singular: Whoever is here can pick up a form. = Whoever is it that is here.../It does not matter who the person is, he or she can pick up a form.
However, if the pronoun 'whoever' is followed by a noun, the noun can be singular or plural, and the verb agrees with that noun:
Whoever the successful candidates are, they will be rewarded.
Whoever the successful candidate is,he or she will be rewarded.
Whoever you are, know that I have always depended on the kindness of strangers...
Whoever they are, they will be apprehended soon.

Hope that helps.
Visnja (better late than never)
What about "the job of watching the fire so it doesn't go out is left to whoever job it is."
Students: Are you brave enough to let our tutors analyse your pronunciation?
... to whoever's job it is is often seen, but it's probably more correct to write ... to whosever job it is.