Editorial / Photoshop Question

[richdlc]

photoshop question for the hive mind...

I'm trying to manipulate a jpeg image for an upcoming book in PS. I want to use the marquee tool to cut and paste part of the image into a new layer, and then adjust the light levels using curves. The trouble is, if I then try and clean it up using the eraser tool I can't merge layers, and I certainly can't save and export the image (seemingly any format but I would prefer to export as a jpeg), I get an error message 'Files can not be saved - program error'. I'm scratching my head here...I understand that the error message is just a warning (something to do with jpegs, compression etc) but is there any way to override it, and still end up with an edited jpeg? I can perform other functions that don't involve layers and save the results no problem...