Melanie Williams' Facebook photo could save your life.

When you see the words skin cancer, you probably imagine a mole with an irregular edge that’s changed colour over time.

This isn’t inaccurate — these are all descriptions associated with potential melanoma. However, there’s a Facebook photo being shared by thousands of people right now that proves skin cancer comes in different forms, and they don’t always match the image in your mind.

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Earlier this month Leeds woman Melanie Williams shared a photo of her thumb, with dark markings clearly visible beneath the nail as if she’d accidentally jammed it in a door.

In her caption, Williams explained the markings grew from nothing over the course of a few weeks, and she assumed they were the result of a fungal infection or a wart. In fact, it was skin cancer — and now Williams is using her experience to remind others of the importance of being vigilant.

"I've been lucky, the cancer hasn't spread and it's being removed next week along with half of my thumb. This may have not been the case if I'd left it and not got it checked out," the 36-year-old wrote.

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"I don't want your sympathy, I want more awareness. Therefore, please go and get any changes or growths on your skin checked out, please tell people around you to get checked as well. Don't leave it to chance or make excuses ... share this or tell someone they need to get checked or make that phone call to the doctors for yourself."

Williams said she will be "fighting fit" in no time, adding that if she'd left the mark any longer she could have lost "more than a thumb." (Post continues after gallery.)

So far, her post has been shared more than 23,000 times — a sign the message has resonated. It can be easy to get lax about having your skin checked, or to dismiss any changes that appear as something benign. Yet early detection is key to successful cancer treatment, so don't put it off.

Williams' photo is also a reminder that skin cancer doesn't always appear in obvious areas, like the shoulder or back. Like the other exposed parts of your body, hands, feet, fingers and toes are all exposed to ultraviolet (UV) light from the sun or artificial sources like sunbeds.

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There's also suggestion the lamps used in nail salons to set gel manicures could pose a small risk of skin cancer because many of them emit UV-A radiation. Williams believes this could have been a contributing factor in her skin cancer diagnosis.

"I've had my nail done with shellac religiously for the past four years, 2-3 times per month. But also previously I've been a user of sun beds. Both of these points make me wonder if these have been a factor in my skin cancer or to blame," she tells The Glow.

Last year, a study of 17 different lamps in US nail salons found the amount of radiation varies considerably from one model to the next, but ultimately a one-off drying session wouldn't expose users to potentially cancer-causing amounts. The researchers say it would take multiple visits — as many as 625 to some salons — for any cumulative DNA damage to occur.

“I wouldn’t tell a patient to stop going unless they were going multiple times a month,” lead author Dr Lyndsay Shipp told Reuters.

The Cancer Council Australia website states "the jury is still out" on this claim as there isn't yet enough research available to prove or disprove it. The organisation recommends further investigation into whether nail lamps could contribute to skin cancer risk.

Speaking to The Australian this week, Cancer Council Queensland spokesperson Katie Clift said, “Exposing any part of the body to additional artificial sources of UV radiation is likely to add to the risk of skin cancer, particularly if that area of skin has already received excessive UV exposure such as sunburn."

Clift also recommended manicure clients "should always wear sunscreen" to lower their risk, while the Cancer Council website states, "Until [nail lamps] are proven safe, avoidance is justified."

RELATED: Look closely - this is what the sun is really doing to your skin.

Checking for changes on the skin under your nails is also wise, as this area is easily forgotten — especially if it's obscured by nail polish.

“Melanoma of the nail is an aggressive form of melanoma and often marks like black spots under their nail are mistaken for a blood blister,” Springwood skin cancer clinic doctor Dora Lee explained to The Australian.

As for skin safety in general, wearing sunscreen, beauty products containing SPF, and clothing with good coverage — plus avoiding long periods in the sun — are important.

Have you ever had skin cancer? What did it look like?