#1) Like Sam, I'm also trying to understand exactly what you are starting with and exactly what you would like to do, so I re-read your initial posting to this thread (i.e., message #1). One thing caught my attention: You used the phrase, "...how do I change/replace one of the layer's images...?". In this phrase, you used the singular possessive form of "layer". This makes me think that you might have more than one "image" on a layer. Is this what you meant, or am I reading too much into the apostrophe?
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#2) If we still aren't understanding your request, I think that probably the best way to help you out is if you would post a PSD file that illustrates what you currently are working with. Please name this and refer to it as , "Current_PSD_file.psd".
In addition, please also post an example of a JPEG image / photograph that you want to add to that file. Please name this and refer to it as , "new_JPG.jpg".
If you would like, for privacy, convenience and/or speed, both your current PSD file, and the new JPG file can be small, simplified versions of what you are actually working with, but if your actual PSD file contains other layers (i.e., either other actual content layers or adjustment layers or layer masks), please include them as well.
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#3) One final question: Presumably, the PSD file that you are starting probably began life as an ordinary photo. Let's name this and refer to it as ,"Original_JPG.jpg".
My question is, "What, if anything, have you done to the original JPG after you put it in Photoshop?" If you have made changes to the original JPG, then could you describe what you did, and post the original JPG, as well.
Don't worry - I'm sure we'll get to the bottom of this.
Cheers,
Tom M