How to replace a part of an image with another image?

Hello

there are so many ways you could approach this, a lot depends on what you already know or are willing to learn...

On the top of my head, ordered from easiest & crudest method to hardest and more precise method:

- google a custom wave brush for Photoshop and draw the sea you need on top of the picture you already have with the appropriate blend mode on your brush. (if you want a white wave use lighten,screen, soft light, or the like.

- use "Select > Color Range..." to make a selection yourself and play with the sliders in color range to tune your result, then use the selection as a mask on either the sky or the sea depending on your preference.

- apply levels dodge & burn the mask's edges to clean up

- you can use blend-if on either the sky or the sea depending on your preference / the exact picture

- you can extract a mask from your sky picture using the Blue channel in the channels palette. You can apply the mask to either the sky picture and invert it to switch between seeing the sky or the clouds.

- you could do the same for the sea picture though it will be more difficult to get a pretty result

- if you need to keep the colors intact (but judging from your example this isn't the case) you could do a small highpass filter on the sea picture (maybe desaturate it) and put it on top of your sea picture with the Overlay or Soft Light blend mode.

Lastly: there are a lot of representations of the idea "sea"... big&wavy, flat expanse of water, shore, white foamy ripples, ...

If you'd post the exact 2 pictures you'd like to blend I'm sure you'll get some more replies out of it. 🙂

Once you remove the parts of blue sky you don't want, it's just a matter of dragging-and-dropping (or copying and pasting) a new image into the existing image of the sky and placing the layer with the new image below the layer with the sky. This will place the new image behind the sky

Two options would be 

• Blend If-settings 

• a Layer Mask based on a Channel 

»Magic Eraser«

I recommend you neber afain use this Tool, it is meaningless for serious work. 

Ülease post a screenshot including the Layers Panel. 

Have you already placed the secong image?