As acts of art terrorism go, it wasn't quite up there with the paint attack on Myra Hindley's portrait or the students who jumped into artist Tracey Emin's unmade bed.

But for the Dimbola art gallery, famed locally for its collection of prints by Victorian photography pioneer J M Cameron, it still caused a sensation.

Incensed by what she saw as its glorification of animal cruelty, Elaine Tubby, visiting the venue with a group of 30 other mature students on a day trip, pulled a pen from her handbag and scrawled: "This is barbaric" across a £400 colour print.

She then slipped back into the group before suffering a crisis of conscience and went to the security desk to confess.

The photograph is an award-winning print by Geoffrey McKillop, part of an exhibition by the British Institute of Professional Photography on its way to Birmingham NEC.

The dramatic image shows a matador with his cloak spread at arm's length as the bull, its bloody neck pierced by spears, rushes towards him.

Mrs Tubby, 63, who lives in Hove, said: "I just snapped really. On reflection, it was probably not the best way to make a protest.

"I'm passionately against bull-fighting and I thought the picture needed a sticker to tell the real story. I didn't have a sticker so I pulled out a pen and wrote a few choice words.

"Naturally the head of the gallery took a very dim view but I think it's important to make a stand.

"I read that one in three bulls doesn't die cleanly and the suffering they go through is simply terrible. People should see that, by attending bullfights, they are keeping the practice alive."

Curators at the gallery and museum on the Isle of Wight have rehung the piece with a short note explaining the unfortunate incident.

Ron Smith, chairman of the venue's management committee, said: "We've never had anything like this and we've been open seven years. Personally I don't like fox-hunting but defacing works of art just isn't on.

"We've spoken to the photographer and he said he would like the print to be replaced so we sent the lady concerned a bill for £400 plus £50 for the trouble it has caused us.

"She sent us a very nice letter apologising and saying she'd never done anything like it before with a cheque for £25 but that's not enough. The police have said they won't pursue it if she coughs up.

"We'll give her a few days to think about it."

Mrs Tubby added: "I've volunteered to buy them a new picture but they're claiming a very inflated sum. It was only a photo."

Mrs Tubby has taken part in three protests against live exports and animal experiments but this was her first stab at direct action.

She said: "I think I'm growing old disgracefully."

Owen Roberts, administrator of Earnley Concourse in Chichester, which organised the trip as part of a course on Victorian history, said: "She's a very respectable lady in her 60s but I think she took offence at this particular work and took it upon herself to deface the picture.

"It's not something we take lightly, of course, but I understand that she is now in the process of making reparations."