At an earlier hearing Stafford Sven Tudor-Miles, 38, from Middlesbrough, admitted five counts of making indecent pseudo-photographs of children.
Judge Tony Briggs was told of the defendant's previous convictions for sex crimes, over almost 20 years.
At Teesside Crown Court, he imposed an indefinite sentence, recommending that Tudor-Miles serves at least 15 months.
At the earlier hearing, the defendant also admitted one count of possessing the images and breaching a Sex Offender's Order.
Tudor-Miles, of Bankfields Road, Eston, Middlesbrough, scanned photographs of adult porn stars into his computer and manipulated the pictures using digital equipment.
He digitally reduced the breast sizes and altered the genitals to make them look like young girls. He added school uniforms to some images.
At an earlier hearing, his barrister argued the pictures were of adults and therefore no offence had been committed. But after failing to have the charges thrown out, he admitted the offences.
They were committed in January and February and came to light after Tudor-Miles' house was raided by police on a separate matter.
The court was told he had asked his GP for help to reduce his urges and sought psychiatric help.
The court was told of previous convictions including in 1997 he was jailed for six years at Birmingham Crown Court after grabbing an 11-year-old girl at knifepoint, tying her up, putting tape over her eyes and then forcing her to commit a degrading sex act on him.
David Lamb, defending, said: "There is no exploitation of a child in this case. There is no evidence of distribution or trading.
"The end result was a rather crude and amateurish portrait."
The judge said: "It is clear to me that fantasising about children in a violent way is still part of your make up.
"In that situation I regret that this is a case where imprisonment for public protection is necessary in unusual circumstances."