Advert On Android Phone

[MTbowels]

I've a HTC One (M7) that seems to have somehow picked up an advert that it displays on start up. As soon as I press the "on" switch at the top of the phone, the normal screen with the padlock appears but in the background there's a full screen advert for a cafe that states "Fancy a bite? Try the Pavilion Cafe". Tapping the advert on the screen does nothing even though there's a symbol of sorts at the top of the screen.

When I open the screen by raising the normal padlock symbol, the advert remains for a second or two before being replaced by my Facebook feed. The advert only appears on start-up and is not seen at all otherwise. How can I get rid of the advert? I have no idea if this cafe is in my locality or not. As the advert looks like wallpaper, I've tried looking for it on my phone but can't find it anywhere.

Thank you

[Old_Geezer]

I think the first thing I'd try is to remove any recent apps you have installed around the time it started doing this. You can get virus checking apps for "smart" phones. I'm unsure how many malware checking apps there are yet.

[TheChair]

I'm with Fender62. If the image isn't active, and has no effect, and disappears when you unlock, I reckon it's lock screen wallpaper, which you can easily change. (The fact that it doesn't appear in your wallpapers gallery is neither here nor there - it's perfectly possible to set a wallpaper from another folder).

[MTbowels]

Thank you all. I've looked under Lock Screen Wallpaper and gone through all the images one by one in the four categories. I can't find the picture anywhere, so I'm assuming it must be in another folder.
I've just tried swiping the image off the screen to the right and it seems to have gone for the time being. I've switched the phone on and off a few times and it's not returned so fingers crossed! I'm still fearful it's hidden in the phone somewhere though.