Hi. I need to extract an image from a photo to make an image on transparent background. Is it possible to make it with Krita?
PS. Original background is complex.

It's very possible, with varying degrees of difficulty and tediousness. If the foreground image is not too complex in outline (hair and fur is difficult) you can use the outline selection tool but on small regions at a time with Mode=Pixel Selection and Action=Add. You'd quickly drag an outline around most of the middle of the objct to be isolated and then do more selections to add small sections of the outer parts of the object. That way you can Undo with not much effort lost if you make a mistake with a small section. After it's all done, you can switch to Action=Subtract if you've accidentally included some background and you want to 'deselect' it.

Another way is to use a transparency mask and make the background areas transparent by painting black on the transparency mask on background areas. Then paint white on it if you make a mistake. You can mix those techniques if you don't like dragging out small selections as described in the first paragraph.

In addition to the methods ahabgreybeard mentioned, there is also an interactive filter available through the G'MIC plugin, as demonstrated in this video tutorial: https://www.youtube.com/watch?v=-f7MzAyCg-Y Though, with a more complex background some amount of clean up by hand is likely necessary regardless.