That's easy to fix bro, instead of pasting layer by layer (I mean the same face+face, body+body, etc), put them all together and use ctrl+t to arrange them together too, this way you only have to use the eraser/loop/select object (or whatever you use) on the background -and having selected the layer to fix- and so there will be no small overlapping parts, then you go removing and putting the visualization of the layer until you fix them and they are not overlapped, also, correct me if I'm wrong but it seems that you do not use the format .png format to open and paste the pic, if it had a transparent background it would be much easier to merge them with the background since they are literally separated to fit perfectly.
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I mean, we can't compare the ease of merging layers with the transparent background.
*no .png (no transparent background)
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.PNG file (with transparent background)
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Also if you allow me to give you a small recommendation, when you put arms try cutting them in half, that way you can resize the layers and they will merge better...
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and you will be able to erase the protruding parts by moving them
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I know you put a lot of effort in making fanarts (or so it seems), more when they are comic type, many times it doesn't seem like it but some of them take a lot of time, without going too far the day I said I was going to take a break from making fanarts, it was because it took me about 1 hour to make 1 fanart and I said: wow, that's a lot of time. The worst of all is that I haven't even uploaded it haha
I'm obviously not an expert but these are little tricks I know to make my life easier when making fanart.
Cheers bro