Hi and thanks in advance.

I have a jpg file with my dogs on it. I want to be able to select the dog only on picture and put it on a separate background.

How do I do that in Krita?

If you've never done any image editing/manipulation/creation work before, there is a lot to learn.

Isolating a natural object from a natural background with the standard selection tools is always difficult, especially if the natural object has hair/fur. Without seeing the picture of your dogs it's hard to say how difficult it would be.

I'd recommend that you use a manual isolation method which will be a bit tedious but is simple and will give good results and also enable you to make corrections at any time if you make mistakes.
If you're not familiar with krita then I'd suggest that you read the manual: https://docs.krita.org/en/

Open the picture of your dogs and add a layer underneath that and Flood Fill that layer in bright green (or bright red or bright blue, just for obvious contrast).

Add a Transparency Mask to the dogs layer, this will be white and so the dogs layer will be fully opaque and visible. You'll be painting on the Transparency Mask so make sure it's the layer that's selected.

Regularly and frequently, Save your image as a .kra file. This reduces swearing in the future.

Choose the Basic-1 brush preset and paint solid black on the transparency mask where there is 'not-dog'. This will make those areas pure transparent and you'll see the contrasting colour layer underneath showing through to clearly indicate where you've made transparency. At the edges of the picture, away from the dogs, you can use a large brush size. As you get closer, you can use a smaller brush size to allow you to get closer to the dogs as you effectively remove the unwanted background.

If you make a mistake (which you will) and you make a part of your dog transparent, you can correct this by changing colour to pure white and painting over the part that you accidentally made black/transparent, to restore its opacity. It's a simple (but tedious) black/white paint job on a Transparency Mask.

As you get to the dog's coat, it gets tricky, depending how fluffy they are and what may be showing through the edges of the fur. If you have a stylus, you can try a Basic_Opacity brush preset and try to manually feather the edges. If you have a mouse, you can try a small brush with shades of grey to feather the edges. If you make mistakes, you can easily go over it again and again until you get it 'right'. It's up to you and how much effort you want to put into it.

When you're satisfied with the isolated dogs, you can put your own background image layer over the initial green contrast layer (under the dogs) to give you the end result you want. Then you can Export that as a .png or .jpg file to send to friends or print out or whatever.

When you've finished, please post a link to the picture of your dogs on the moon, or whatever it is you've done.

Yea I know, i read through some of the manual but Im not trying to be professional here...just need something from time to time.

The tranparency mask when using the basic 1 black it changes the entire mask, not just where I clicked.


Thanks

It sounds like you still had the Fill Tool selected when you clicked on the Transparency Mask. I forgot to say to switch to the Freehand Brush tool for manual painting on the mask.

Here's a video tutorial that might be useful for simplifying some of the tedious steps: https://www.youtube.com/watch?v=-f7MzAyCg-Y
That method uses a filter in the G'MIC plugin at the beginning, which is not available in the macOS version of Krita in case you're using that.