The main layer of a picture that you open up in photoshop is usually locked by default. If you double click on it you will be able to unlock it.
If the picture is a lot smaller when you drag it to the other image its because of DPI.
Say you have document open at 300 dpi (press quality) and then you try to drag an 8.5 x 11 picture that was originally at 72 dpi (Web quality) its normal that the picture will shrink a lot when you drag it since it does not want to resample the image. since you only have 72 dots per inch in the original image and you have a whole 300 in the new one the image comes out smaller.
So you have 2 choices
Either find a Higher resolution image (what i would recommend)
or manually make it bigger. (by doing this, Photoshop will resample the image and you will lose quality.)
thank you pliff !!! you have been alot of help.