Example: image blend
I'll go for corny on this one :p
heres a pic of me:


on top of that as a new layer is a bunch of roses:


I'll lower the opacity of the rose layer so I can get a good view of the pic, and then I'll get my freehand selection tool. I'll set the selection type to point to point, the feathering to 4 and use anti-aliasing as well. Now I do a rough selection of me


I rurn up the opacity of the rose layer again, choose Masks -> New -> Hide selection. And it now looks like this:


I remove the selection (CTRL+D) and start working on the mask by choosing Mask -> Edit Mask.
I grab the paintbrush and the white color and start drawing where I want more of the rose layer to show.


Then I get the black color and draw where I want the pic beneath to show.

I am going for a sitting-amidst-roses effect here...
I'll finishe the mask off by using the soften and the smudge effect in the retouch tool, but you can use any effect and tool you'd like. After a bit of softening and smudging I get this as my final result:



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